Reminisce is a dreamy way of saying "remember the past." If you're swapping old stories with friends and remembering all the silly things you used to do, then you're reminiscing.
Reminiscing is all about happy recollections and thinking back to stories from the past. It's hard to reminisce about the old days with someone you don't know very well or haven't known very long, since a key part of reminiscing is a shared history. But just about anyone can reminisce about going elementary school, learning to tie shoelaces, or getting that first job.