Hello and welcome to the forum 
When you draw something in a frame, it is then a ‘keyframe’ and the frame in the Timeline docker will be blue to indicate that it has content.
To the right of that frame, you’ll see a horizontal blue line extending into all future frames.
These are called ‘Held’ frames and that indicates that they will be rendered with the same content as the content to the left of them, all the way back to the previous keyframe.
This is very useful.
If you want the Held frames to stop, then you select the frame at which you wish them to stop and do right-click → Create Blank Frame and that will put an empty frame there and there will be no more Held frames after that, until you draw another keyframe somewhere to the right or in that freshly created empty frame.