Cold calling is an act of just calling a raise made by another player if you haven’t made any aggressive action in the betting round.
This term is usually used to describe someone calling a re-raise. For example, let’s assume a player under the gun opens the pot by raising, and the cutoff decides to 3-bet. If you are sitting on the button and decide to call that re-raise, you are making a cold call since you have no chips invested into the pot and decide to call a previously made bet.
The same can be said for a similar situation when there is only one raise in front of you. Following the previous example, if under the gun player raises, and everyone folds to you on the button where you decide to call, that is also considered a cold call.
Usually, you should avoid having many cold calls in your range and opt for 3-betting instead, but it is fine to mix your strategy a bit and make this play from time to time.