Back clipping is when you clip the rope backward so the side the climber is attached too is on the inside of the carabiner rather than the outside.
As you can see from the above diagram the rope when back clipped has the possibility to pull back on itself and unclip from the carabiner which means you will be taking a pretty big lead fall. If this does happen don’t panic just collect yourself and reclip the draw the correct way. If you have already climbed past it without noticing as long as you clip the next draw correctly you can ignore the back clip however its good practice to ensure you don’t back clip in the first place.