Terms whose variables (such as x or y) with their exponents (such as the 2 in x2) are the same.
Examples:
7x and 2x are like terms because they are both "x".
3x2 and −2x2 are like terms because they are both "x2".
But 7x and 7x2 are NOT like terms (the exponents are different), they are unlike terms.
Like terms can be added together.
See: Term