Symbol used online for 3 things usually.
1. Bolding or emphasizing a word where font types are unavailable.
2. A way of setting off a word that gives what you're writing a "tone" without actually leading the reader to believe that you're saying the word.
3. Making a correction to a previous mistake.
In order of the definitions of asterisk:
1. I *really* want to come over and rotate your tires.
2. *grin* If you gt what I mean.
3. *get (or get*)