Bleed
Bleed is the extra edge of colour you add past the cut line, so the printed colour reaches the paper edge.
In a simple example
Your flyer has a brown background, so you extend it 3 millimetres past the edge, and the cutter trims it cleanly.
Why it matters to you
Without bleed, a small white line can show at the edge after cutting.
Do not confuse with
Bleed is cut off in the end. It is not a margin you can see.