Music Technology Terminology .. what does Zero Crossing Points mean?

[Christina -]

i recently submitted a piece of music to film where i had to create sound effects
the notes i received about my piece afterward said

''this piece suggests that edits have not been observed at zero crossing points, and that fades have not been used''

i have emailed my tutor and asked him to explain what Zero Crossing Points means as i need o do a re-submission of the work (dont want the same problem to bring my grade down) but he still has not replied.. im hoping that someone on here can help me out.
Does anyone know what it is??