like terms will have exactly the same variables
so 5a and 2a are like terms
examples :
3b and 4b are like terms
3b and 4a are not like terms
2a^2 + 3a^2 are like terms
2a and 3a^2 are not like terms
3ab and 4ba are also like terms...even though the variables are switched
6b^2a and 6a^2b are not like terms