Input lag can be described as delay between input and output. It occurs mainly during fast scenes where there is a shift between images that comes from the graphics card (or the computer in general) and the real image at output.
Imagine a situation where there are two monitors connected to one computer. On the PC, you run a stopwatch and shoot the monitors in slow-motion mode. If you make a video, you will find the monitors differ slightly from the input lag and the stopwatches will show a different time.
