If you see a @ at the beginning of a line, or several lines in a row, and nothing after it, it's a place holder for a line that's too long to fit on the screen. If you scroll down the file farther so that you have more room, the whole line will be displayed.
Now, if you're talking about ^@, that's how vi represents the null character, i.e. ASCII code 0. Of course, unless you're opening binary files in vi, you probably won't see that too often.