What makes the rough endoplasmic reticulum rough?

The ribosomes on the surface.
The rough endoplasmic reticulum (ER) is called ‘rough’ because it has organelles called ribosomes attached to the surface.
Ribosomes are the organelles that turn mRNA into . These proteins are initially long strings of amino acids. This string has to be folded in the right way to form a functional protein. The rough ER helps with the folding of these amino acid-strings and at the same time prevents their degradation by enzymes in the cell.
Because of this cooperation, the ribosomes can be attached to the ER. The ribosomes can then synthesize the amino acid string straight into the ER, making the process safer and more efficient.