Sit on the edge of your seat' means you are expecting or excited to watch something happen.
If something keeps you on the edge of your seat, it is very exciting and you want to know what is going to happen.
Hope this helps.
Cheers
Note: 本人已在海外生活了数十载.为了慎重起见,我的答案都与老美或老英交换过意见,你放心使用吧.如有任何问题,可补而述之.