Old CRT televisions displayed images by shooting an electron gun at a phosphor coated screen. When the electron from the gun hits the phosphors on the screen it causes them to glow for a very short period of time.
Interlacing was invented because by the time the gun got to drawing the bottom portion of the screen, the phosphors at the top had already stopped glowing and the top part of the screen was getting dark.
They started interlacing, or drawing every other horozontal line of picture (say the odd numbered lines) and then the gun would return to the top and draw the lines it missed in the first pass. This allowed the picture to stay bright across the whole screen.
However the delay in time between the first scan (or field) and the second scan (field) meant that the content in the picture could change from the first field to the second. So each "frame" of video was broken into 2 "fields"
In interlaced video the fields are both included in the frame, and this can appear as little jagged lines, especially in areas of movement.
So to answer your question, field stripping is the process of removing one of the fields and compensating for the lost field, usually by copying the line above it. You effectively loose half of your resolution this way. This is usually done by selecting "remove interlacing" in your preferred software.
