If f: G-> H, then
The kernel, K, of f is a subgroup of G - {x in G such that f(G)= eH}
The Image, I, of f is a subgroup of H- {y in H such that y= f(x) for some x in G}
1) K is a normal subgroup of G.
2) G/K is isomorphic to I.
Do you understand what "homomorphism" and "isomorphism" mean? Do you understand what a "normal subgroup" is and what "G/K" is?