It is a letter written by someone other then a professor.
USF requires 2 letters written by someone outside of your professors. This could be a research supervisor, physician, peer, etc. Someone who has seen you in action and can talk about more then your academic qualities.
Most schools require 2 science professors and 1 nonscience professor. Some schools like FSU want a peer to write a letter. Others like USF want two nonprofessor letters that are written by supervisors, physicians, friends, etc. Someone who knows you real well.
Some schools even ask for letters specifically from a professor from a particular subject. One school that my friend applied to wanted letters specifically from a physics professor, bio professor, and chemistry professor.
It really depends on the individual school requirements. If you don't meet the school requirements your application won't be complete, because you really need to give them everything they require for the application. This will be specified when you get your secondaries.
