Animate masking help

I don't know what is going on but I tried masking my animation to block certain parts from showing and it's not working. This is my first time trying to mask and this program is giving me hell. Any help would be great. I want to mask two things. My animation is a bunch of bars moving up and off the screen of a phone, the problem is I can't move the bars off the screen without the animation showing the bars leaving off the stage so I was going to mask it but this masking stuff is giving me trouble. I tired looking up how to mask but it is turning out way too complicated. I tried using a shape to mask the area. I thought I did it right but everytime I try to preview or export the animation it is not turning out at all. It's extrememly frustrating. why doesn't adobe just make a simple mask button you click to mask and BAM you're done. I swear they love to make stuff more complicated than it should be. Anyway, if someone could please help me I would appreciate it.