An equation is a mathematical sentence that contains an equal sign.
To understand this further, I'll show an example.
Suppose that we had a balance and a bunch of blocks for reference. If we put the bag on one side of the balance and put a number of blocks on the other side of the balance, they even out. If the number of blocks on the right side of the balance is 8, this means that the number of blocks in the bag is 8. We can write this as X = 8.
X = 8 is an example of an equation since it's a mathematical sentence that has an equal sign in it.