The cathode ray tube (CRT display) works like an old giant tv. The electron beam sweeps across the screen in a zigzag fashion. It start at the left top side and scan each line from left to right. At the end of the line it jumps back down to the next line and continues on that line. When all lines are done it returns to the upper left corner of the display and start again. A interlaced display mode works the same but does the whole display in two cycles. The first is even-numbered lines and the next in odd-numbered lines.