In a situation where you open, your partner responds and then your Right Hand Opponent overcalls, it's valuable to agree what a double by you would mean. Here are 4 possible meanings:
1. Penalties
2. If your opening 1NT is weak: a strong-no trump
3. Take-out and better than minimum
4. A "support double" showing 3 cards in partner's suit
Type 1, penalty is what we used to play when dinosaurs roamed the Earth. It brought in the odd 800 which was handy at teams, but just isn't common enough to work well at pairs.
Type 2 is probably the most common amongst weak no-trump players and is what I would expect a player in the UK to have if this came up without discussion.
Type 3 has merit and in practice is not really that different to option 2.
Type 4 the support double was invented by the top US international Eric Rodwell back in the 1970's and is massively popular amongst strong no-trump players, for good reason, because it allows partner to tell whether you have supported on three or four cards in their suit. There are people who combine it with a weak no trump, but it does leave a hole on many type 2 hands and for that reason is not usual amongst weak no trump players. It's a pity because in its own right it's an excellent gadget.
Finally, here's a hand that involves a type 2 double (this is Douglas Bridge Club, 17 March board 7)
Ultimately East-West got a bit too high in competing but the focus today is on knowing what your double means, so we won't worry about the rest of the hand