Cue bidding is used by most expert level partnerships, and I highly recommend it for all partnerships - its not difficult to understand and it is worth the effort to learn.
Clarify this with your partner
What does a cue bid show? That may sound like a simple question, but the answer varies greatly depending who you ask. I'll give my recommendation and also look at where there might be confusion.
What is 1st or 2nd round control
First round control is an Ace or a void in a suit (if you have agreed a trump suit). Put simply, first round control simply means "I can win the first round if that suit is played!"
Second round control is a King or a singleton, similarly this means "I can win the second round of the suit if that suit is played!".
Cue bidding, at least initially (other articles will be coming with exceptions), shows EITHER 1st or 2nd, so we are left guessing to an extent, but it is a guess that is not normally a problem.
The main idea of a cue bid
If we are thinking of slam, we don't want to be wide open in any of the suits. If for example, the opponents can cash the Ace and King of diamonds, taking us down in our slam, its a good idea to try to avoid it, for example take this auction
Example 1.
Let's assume for simplicity that 3♠ showed slam interest (not a typical modern treatment). Lets look at the rest of the auction
4♣ - I have a reasonable or better hand, if I had a bad hand I wouldn't cue bid altogether (one aspect of cue bidding is that it shows a good hand). I also have 1st or second round control in clubs (in this case a singleton).
4♥- Partner is saying - I also have a good hand, and I have 1st or 2nd round control in hearts, but NOT diamonds. Why not diamonds?
So in example 1, partner has denied an Ace or King of diamonds, and doesn't either have a singleton or void - Since we do not either have a control in diamonds, we simply sign off, we are absolutely sure slam is a bad idea. This is the full hand
As you can see, we could simply lose the first 2 diamond tricks, which we have diagnosed, and successfully stayed out of slam.
Should we always cue bid?
No, with a bad idea it is good to simply sign off, conveying that message clearly to partner. It is very rare that partner will be able to bid slam despite you having a total minimum hand. So, for example
Once again lets assume 3♠ was a slam try hand. In this case its not a good idea to cue bid your diamond King. It is much better to convey to partner "I have a terrible hand, if that is relevant to you, sign off!". In an experienced partnership, the players are aware of what type of hands might sign off, for example its common to make the rules
1. A minimum hand
AND
2. A hand without many Aces and Kings (these are often the key ingredient for slam if partner has an appropriate hand).
In other words, we don't sign off with all minimum hands. It needs to be a minimum AND a bad hand - lots of queens and jacks, not many Aces and Kings.
Essential cue bidding rule: "Bids below 3NT are natural"
This is an essential rule, particularly when a minor is agreed, for example
Partners bid of 3♥ is natural, showing values there. It is not a cuebid. Since 3NT is one of our favourite contracts, bids below 3NT should basically communicate the message "Is 3NT a viable contract rather than 5 of a minor". We have no problem bidding 3NT with those cards in spades, likely we have a double stopper.
So, Cuebids only apply when
1. We have agreed a major
2. Generally only on the 4 level and higher. (there is an exception people play where they treat 3♠ as a cue bid, when hearts are already agreed. Why? Because otherwise to find out whether partner has a cue bid, we would have to bid 4♠, which forces us to the 5 level! Don't worry too much about this if its new to you, just have it in the back of your mind in case partner wants to play that).
Main source of confusion
1. In the past it was common for cue bids to only show first round control (Aces or voids). In most auctions this is no longer considered the modern way, my recommendation is to use it as I've talked about above - 1st or 2nd round, and also as showing extra strength or in some way a decent hand.
2. Does it show extra strength or not - some people always cue bid, they say to not cue bid denies the ability to (no control in the suit). As mentioned I think it should also show extra strength, take for example this hand.
For example
5♣ here is already a contract that may need some luck, you only have a combined say 23 points. Just bid game, there's no reason to exaggerate your hand by cue bidding.
3. It only applies on the 4 level or higher (subject to the one exception)
Overall
I highly recommend learning cue bids and practicing them. Always keep it simple and remember you need to be thinking about slam in order to cue bid, with a very bad hand rather just sign off and let partner know your hand is bad.
Cue bids are so widely used that the benefit of learning them properly is
1. Partner may want to play them
2. To understand what the opponents are doing, in defence often it is helpful to know what the auction meant.
Read next: Best slam bidding conventions · Blackwood and Roman Key Card Blackwood (RKCB) · 5NT Pick a Slam: Practical Partnership Agreements