It is a place to look up things when an equation requests information. Don't try to understand the problem statement. Trying to understand a problem statement can quickly lead to confusion, which results in flailing at the problem rather than solving it. The understanding of a problem evolves as it is solved. In a nutshell Starting a problem solution is the simplest, least time consuming step in solving a problem.
Problems solve themselves.
A problem is understood by solving it, not by pondering it.