A substitution is when one thing or person is put in the place of another. The word covers both the act of swapping and, quite often, the thing that goes in — so 'the substitution took ten seconds' and 'this is a substitution for butter' are both normal. In sport it is the everyday word: when a tired player comes off and a fresh one comes on, that is a substitution, and coaches talk about 'making a substitution'. In cooking and shopping it means using a different thing because you cannot get the first one — if a shop sends you a different brand, they made a substitution. In school subjects, teachers use it for the act of putting a number in place of a letter: 'solve it by substitution'. It sounds a bit formal or technical. In ordinary talk people usually say 'swap', 'change', or 'they put someone else in'. Notice that a substitution is meant to do the same job as the thing it replaced — that idea of 'doing the same job' is what separates it from just 'a change'.