Every commit in git has at least one parent (except for the first/initial commit). A commit's parent is the previous one.
C1 <- C2 <- C3
C1 is the initial commit. C2 is the second one. C1 is the parent of C2. The same goes to C3.
A merge commit is a special commit in the sense of the number of parents.
C1 <- C2 <- C3 \ .. C4 <- C5 <- C6
C6 is a merge commit. It has two parents, C3 and C5. If you merged the two branches (commits) when you were at C5: C5 is said to be Parent 1 (first parent) and C3 is Parent 2 (second parent).