An affix is a morpheme that is attached to a root (or stem) of a word to form another word.
For example, take the word reason and add two affixes:
un + reason + able
Prefixes and suffixes are common types of affixes.
A prefix is an affix which is placed before a root.
un + kind = unkind
re + act = react
mis + lay = mislay
A suffix is an affix which is placed after a root.
reason + able = reasonable
look + ing = looking
danger + ous = dangerous