Raster
A raster picture is made of tiny coloured dots, called pixels, so it blurs if you enlarge it too much.
In a simple example
A photo of a cake is raster. If you stretch a small copy onto a big poster, it looks fuzzy.
Why it matters to you
Photos are raster, so you must keep a large version for printing.
Do not confuse with
Raster is made of dots. Vector is made of shapes.