Over-the-top, or OTT, is an industry term that applies to streaming media.
Before people started using the Internet to watch TV, many people had a subscription to cable TV. The cable company has a single cable that goes from their end to your home. The cable company decides what you watch and what time it airs (with added options for "on-demand" programming). The cable company pushes content to you.
In OTT, you decide what you want to watch and when you want to watch it. Then you stream that content over the Internet, or on top of the Internet platform that you use for other things such as work, email, gaming, etc. Each individual pulls his own content in his own stream rather than being restricted to one per cable box.