It typically indicates a lack of, absence of, or opposite of the word it precedes.
Here are some examples:
* Negative: Not positive
* Neglect: To fail to care for
* Negotiate: To discuss in order to reach an agreement (the opposite of fighting)
* Negligible: Too small to be considered important
* Negation: The act of denying something
Keep in mind that "neg" isn't as common as "non" or "un" when forming negative prefixes, but it does appear in some words.