You can look up the definitions on Wikipedia. A subset of vertices is called an independent set if no pair of vertices are connected. Maximum independent sets in Petersen graph have 4 vertices (this is called the independence number).
A set of pairwise adjacent vertices, also called a clique, is a subset of vertices such that every two vertices in the subset are connected. I believe the maximum clique in Petersen graph has size 2.
Note that "a set is independent if and only if it is a clique in the graph’s complement" (Wikipedia).