On a basic live, that simply means acting like one's boss -- the manager who one works for. Depending upon the context and who says, "like a boss", it could be Negative:
1) "bossy" (liking too much to tell people what to do)
2) acting in charge when one really is not
Positive:
1) embracing one's authority, leading legitimately
2) being proactive
The expression is most often used negatively, especially if there is no boss/employee relationship between the two people.