Before editing video on computer was possible, most editing was done on linear editors --- these work by having essentially, two video recorders. The first would hold the video that you are editing together, and the second would be used to find the next clip that you want to record. Find a clip, record it, find the next clip, record it. This is called linear editing because the medium of tape forced things to be edited sequentially. If you have edited 10 minutes worth of footage together, and decide that you made a mistake after five minutes, you can't just "push" the last five minutes around to add space for new clips.
Non-linear editing is by far a more flexible way of editing. You can place clips anywhere within your final video (in Kdenlive, this is represented via the timeline), and you can move them around as much as you want, rearrange them in a completely different order. Best of all, at any point during this you can press play and (with a sufficiently powerful computer) watch back what you have done there and then.