(Any excuse for a photo of David Beckham).
"Substitute" means to put in the place of another.
When a substitute comes onto the pitch in football, he is normally replacing a tired or injured player. In maths, we replace a letter with a number.
We use substitution all the time in maths. A good example is when we solve a quadratic equation using the quadratic formula. The quadratic formula is so important that some (crazy) people even get it tattooed on their body!
Remember, substitution follows the rules of BODMAS – do indices before multiplication. This website has a good summary of the rules, including practice questions.
Here's the worksheet we did in class, including answers.