In this it helps that both the underlined word "precursor" and a key word in the definition have the prefix "pre". "Pre" means before. This sentence is saying that it is important to take algebra before calculus because the skills one learns in algebra act as a basis for the equations and skills that one must complete in calculus. It's important that students who take algebra earn a passing grade to show their competency before moving on to a more complex course like calculus.
