In more detail
Normal colour vision uses three kinds of cone cell. A deficiency in the red-sensitive cones is called protan, in the green-sensitive cones deutan, and in the blue-sensitive cones tritan (rare). Mild forms (anomalous trichromacy, such as deuteranomaly) shift colours; severe forms (dichromacy, such as deuteranopia) remove one axis of colour entirely. Roughly 1 in 12 men and 1 in 200 women have a red-green deficiency, and most of them have a mild form.
Why it matters
Red and green status lights, chart lines told apart only by hue, and error fields marked only in red can all be unreadable to a large share of users. Never use colour alone to carry meaning: add shapes, labels, icons or patterns, and test designs with a simulator. Good Colour contrast in lightness also helps, because lightness differences survive every kind of colour deficiency.
Example
Pure red #ff0000 and green #008000 look very different to most people, yet their contrast ratio is only 1.29:1 because their luminance is almost equal. With the red-green difference removed, they can look like two similar dull yellow-browns. Adding a ✓ and ✗, or the words "pass" and "fail", fixes the problem.