Something or someone that takes the place of another is said to be a substitute. You may be sent into a game as a substitute for an injured player, or you might substitute chocolate chips for blueberries in a pancake recipe.
Substitute can be used as an adjective to describe something or someone that takes the place of another. Teenagers are notoriously hard on substitute teachers, sometimes giving them fake names or wrong information about classroom work. Substitute can also be used as a verb. So if you are asked to substitute for a teacher, you probably shouldn't believe everything the students tell you!