A prefix is a part of a etymology that modifies are word at the start of it: i.e. reconsider, involuntary, uncover.
A suffix is the same thing but it modifies the end of a word: i.e. walked, bigger, reversible.
De is a prefix meaning "down, away" or "denoting removal or reversal". In this case it is referring to demagnetizing something, or removing the magnetization.
Ize is a suffix meaning to "make or become". This is referring to an object becoming magnetized.
