Galangal is a close relative to ginger. There are several varieties with the most common are "greater galangal" and "lesser galangal." Lesser galangal roots look like long fingers, whereas greater galangal (shown) looks like fresh ginger. Both are used in South East Asian cooking.
If you don't have galangal here are your best options for a substitution, per 1 tablespoon chopped needed:
Look for fresh galangal in Asian grocery stores. There is a seller on Amazon that ships Fresh Thai galangal. You can substitute dried ground galangal, but it is not very good, see substitutes. You may also find the roots in the freezer section of Asian grocery stores but you're better off buying fresh and you can freeze your own if you want a little for future use.