Using the Mail Monitor plugin

The yellow balloon thing is a tooltip, meaning it only pops up when you hover your mouse over the icon. If you have mail (usually you can tell at a glance by looking at the icon - if it's animated, you've got mail), you can hover your mouse over the icon, and the tooltip will tell you which account recieved mail, how many messages, and how big (file size) they are. Otherwise the tooltip only shows the caption for the plugin (default: Mail Monitor).

By the way, I believe the tooltips default to a rectangular shape instead of the balloons. To change it, open up your TLB Settings, and select the tooltip style you want from the drop-down list on the General Options tab. Personally, I prefer the balloons because they go better with the comic book style theme I'm developing