To Matter

1. (intransitive) To be important. [from 16th c.]
2. (transitive, in negative constructions, now England regional, Caribbean) To care about, to mind; to find important. [from 17th c.]
3. (intransitive, medicine, archaic) To form pus or matter, as an abscess; to maturate.

it doesn't matter

no matter (“in spite of”)

(be important): signify