Vibrato is a musical effect, produced in singing and on musical instruments by a regular pulsating change of pitch, and is used to add expression and vocal-like qualities to instrumental music.

There ya' go. Does that answer the question?

The music teacher I was talking about was not in fact a singing teacher and he made the remark to indicate that some singers (not all) would (mis)use vibrato as a means to cover up poor intonation.