If you substitute one thing for another, or if one thing substitutes for another, it takes the place or performs the function of the other thing.

A substitute is something that you have or use instead of something else.

If you say that one thing is no substitute for another, you mean that it does not have certain desirable features that the other thing has, and is therefore unsatisfactory. If you say that there is no substitute for something, you mean that it is the only thing which is really satisfactory.

In team games such as football, a substitute is a player who is brought into a match to replace another player.