Contrast is how different two nearby things look. On a screen, a dark button on a light background has more contrast than a pale button on a nearly matching background.
Enough contrast helps people notice important controls and read text. Imagine trying to read a gray note written on a slightly different gray wall; the words may be there, but they are hard to see.
Color is not the only way to create contrast. Size, shape, and spacing can help too. Check that important information remains clear for people with different vision needs.