The resolution is the primary measure of the crystal quality and accuracy of the structure model.

Low resolution:
 ~ 3-5 Angstrom - overall shape, side chains not resolved anymore
Medium resolution:
 ~ 2.5-3 Angstrom - side chains can be distinguished
High resolution:
 ~ 2 Angstrom - side chains, waters, ions, small ligands
< 2 Angstrom - alternate side chain conformations (rotamers), holes in aromatic residues
< 1.1 Angstrom - some hydrogens