When someone substitutes something, they use it instead of something else.
Substitutes can be good if the new thing is just as good as the old thing, but they can be bad if the new thing is not as good.
Examples:
She substitutes apples for oranges in the recipe.
We need to find substitutes for the missing ingredients.
He substitutes his toy for mine.
The teacher substitutes for the regular teacher when she is sick.
In the game, the substitutes play when the regular players are tired.
The robot substitutes for a real person in the movie.