"Protected class" is a term used in US law to indicate a group of people with common characteristics who are protected from discrimination based on that characteristic in certain circumstances. It's illegal to discriminate against people over 40 in the US for employment purposes because "age over 40" is a protected class in employment law via the Age Discrimination in Employment Act of 1967
See[1] for a list of the protected classes
One would assume a hiring manager would have a familiarity with the laws around hiring. Especially since many states (and maybe the US?) are required to post information about those sorts of laws in a visible place in places of employment.