Bleed is extra artwork extended beyond the final trim edge of a printed piece — usually about 1/8 inch — to account for the small amount of natural shift that happens when a sheet is cut.
Without bleed, any slight misalignment during cutting can leave an unwanted thin white sliver along the edge where the background color or image was meant to run all the way to the border.
Print-ready files should always include bleed for any design element meant to touch the edge of the piece — a printer will typically reject or flag a file that's missing it.