Something in door

so i look all over the door wondering where the hell it could come from. and i found one on the front of the window looks like somekind of slider. the one on the car looks newer than the one i found in the bottem of the door. also it looks like the one that i found floating around was drilled. so maybe he had to replace the window and just left this peice in the bottom of the door? or is there supposed to be two on the window? just want to know so i don't break anything in the future cuz its missing something. the other question i had is that clearish plastic i had to peel back to get in the door do i NEED that or can i take it off? i'll put it back on if i have to but i don't like dealing with that black junk they put on it its hard to get out of everything

