To nibble is to eat in tiny bites. If you nibble your chocolate chip cookie while your sister gobbles hers quickly, yours will last much longer.
Your pet rabbit probably nibbles on the carrots you give him, and he might even nibble, or bite very softly, on your fingers. Any one of these gentle or small bites can be called a nibble. The word probably stems from the Low German nibbeln, "to gnaw."