Double (X) is a really flexible bid. It takes no room on the bidding ladder, but suggests your side could be in the auction.
Double was originally used to increase the penalty when opponents bid too much (called a penalty double), but it’s unlikely to work that way at low levels. So, most doubles are now known as takeout doubles because they ask partner to bid.
Double shows an opening hand (13+), with shortage in the suit opened, and the ability to support the other three suits. Count your shortages when you're deciding whether to double, but keep in mind they’ll work well if you play in a suit, but they won’t improve the hand in no trumps. So be happy about your shortage, but be a little careful too.
You hold ♠KQ75 ♥AJT2 ♦5 ♣K974 and the hand on your right opens 1♦. You have a good hand with 13 points, but you can’t overcall, as you don’t have a good five-card suit, and who knows which suit would be your best spot? That will depend on partner’s hand. So, say “Double” and that will kill three birds with one stone!
It’s essential to answer partner when they double even if you have nothing (if the next hand passes), because if you pass as well, the contract will become 1(of the suit opened) X, and that will give you a bad result if it makes even seven tricks, as the score will be more than doubled.
It’s less important to have a shortage in opener’s suit when you hold more points. If you have a good suit, but think it’s too strong to overcall, (17+ points), you’re better to start with “Double”, and then show your suit over partner’s response. This way the hand won’t be passed out ( because a simple overcall is not forcing).
For example, you hold ♠A4 ♥Q52 ♦AK7532 ♣A9 (17 high card points, and 2 length points for the six card diamond suit). Your right hand opponent opens 1♠. Bidding 2♦ may be passed by partner, so double first, and when the bidding comes back to you, show your diamonds. Parter will understand that you have a hand like this, and will try to bid game.