Topics are like email subject lines. The big difference, though, is that they're really short and lightweight. Two or three words will do it. Don't overthink 'em -- you can always edit the message later!
Why bother with topics? Well, two reasons: it makes conversations clearer (imagine if email didn't have them!), and it lets you more efficiently catch up on what's happened while you're away -- read the topics that are relevant to you, and ignore the ones that aren't!
