You possibly noticed that in many movies - especially shot with home camcorders - in episodes where fast movement is recorded, the image looks blurred and stripped. This is because many cameras divide each frame into even and odd lines: first the even lines are recorded, then the odd ones. Hence, each frame you observe on your monitor is actually comprised of two half-frames - "even" and "odd" ones.

Half-frames are connected with each other line by line - that is, the first line of the first half-frame - the first line of the second half-frame; the second line of the first half-frame - the second line of the second half-frame; etc. As a result, if the frame contains fast enough movement, then on the second half-frame the moving object has already moved to slightly different position compared to the first half-frame, and at the connection points the "stripes" effect occurs.

Deinterlace is a process which allows to avoid the undesirable effects while connecting the two half-frames.