Sounds like you have turned on the safe area overlay. There's a button in the top of the preview area with a little pulldown arrow. Click on this and turn off 'safe areas'.

The use for these is if you are producing material for old CRT televisions. Most old screens don't show the entire picture, but can cut off 10 to 15% of the images around the edges. This is called "overscan" The outer line shows what is likely to be visible on most sets, while the inner line is the 'title safe area'. All titles should be inside this inner line to guarantee they won't be cut off.

Modern flat screen TVs still have some overscan, but usually much less than 5%.