Ais a horizontal or vertical bar that appears when the contents of an area may not be visible completely on the screen.

The answer is a Scroll bar, the description fits perfectly because you need to move it to see more of the content you are viewing, and it wont be completely. when the item does appear completely on screen then the bar is relatively large and there is  no need for scrolling, however the opposite occurs during when you cannot see all of it.

Just trying to get some points : p sorry bro