The signs matter for the sake of operations subtraction and addition.
Similar terms are terms with exactly the same literal coefficients, meaning same variable (represented by letter) and its exponent.
Examples:
1) -xy², 4xy², -¹/₃xy² are similar terms. They have the same literal coefficients xy².
2) 4x²y, 5xy² are both positive terms but they don't have the same literal coefficients. x²y and xy² are not the same. These terms are not similar.