To understand what an accidental is, you must first understand what a key signature is.
That is answered at: What is a key signature?
.. but briefly, a key signature is a set of markings telling you which notes to always play as sharps or flats. For example, the key signature for F major consists of a ♭ in the B position, meaning "Whenever a B appears in this score, play B♭".
An accidental points out a note which is an exception to the key signature. It is a note with a ♯, ♮, ♭ next to it, to say "although the key signature says otherwise, for this bar only, play it like this".