Most professor salaries are on 9-month appointments, for the academic year. The three summer months usually come from grants, teaching summer sessions, or consulting.
However, a professor can choose to spend the grant money on student support rather than summer salary (thus forgoing their own salary), which is what I assume the author is referring to.
However, a professor can choose to spend the grant money on student support rather than summer salary (thus forgoing their own salary), which is what I assume the author is referring to.