It’s your job to remove these lines from the screen by tapping on them. You can’t just jab them willy nilly, however. You can only clear them in a particular sequence, and you have to work out what that sequence is through careful deduction. While the stages are arranged in a particular sequence, with the simplest at the beginning and new mechanics introduced as you progress.
The overall objective in is to clear the screen of lines. Like a construction worker picking up steel girders, you can only move the ones that don’t have another one on top of them. In the first couple of stages this is very straightforward once you understand what you’re supposed to be doing, but pretty soon the stages get complicated, with lines turning corners and overlapping other lines in places, while being overlapped in others.