The peaks in the RDF tell you where the atoms spend the most time. If the function goes to zero at some point then you know that any two atoms are never at that distance from one another. A typical RDF is zero up until a certain distance and then has a large peak. This is because the potential diverges to infinity at small interatomic distances, hence any two atoms can not be within a certain distance of eachother.
A liquid typically has a somewhat smooth RDF that fluctuates around 1 after the first large peak. The RDF of a system in the solid state has many sharp peaks with the RDF going to zero between them.