The <++> is a placeholder. It's a function of the IMAP plugin included in vim-latex. You jump to it by pressing Ctrl-J.
Consider this example. When you type in "EDE" (the shortcut for the description environment) in insert mode, the following shows up:
This puts in the environment, with placeholders where you need them; first your cursor is placed where the label would go, so you can fill it in. The next thing you'd type in is the rest of the description, so instead of moving over to it, you can just press Ctrl-J. Then, it leaves a placeholder outside the environment, so you don't have to move out of the environment.