A subgroup N of a group G is a subset of the set of elements of G (in which the group relations still hold.) If we multiply the elements of N by an element a not in N we get a set of products, distinct from N, and of the same size as N. This set is called a coset. If there are elements not in N or the coset we can multiply N by one of those and get a new coset, distinct from the first two sets. In general it matters which side we multiply on: . If aN = Na, (meaning not that individual products are equal, but that the set of products are equal), then N is called normal or invariant. This is equivalent to being self-conjugating, where g-1Ng = N, i.e. g-1n1g = n2 where n1 and n2 are members of N. (See bottom of I)
Illustration: given Dih3, one subgroup is I, s, s2. We can multiply on the right by t, which is not in the subgroup, and get t, st, s2t=ts. Or on the left to get t, ts, ts2=st. In this case it's the same coset, permuted a bit. If we multiply by st we get the same set: st, st=ts, s3t=t. On the other hand, we can consider the subgroup I,t and multiply by s to get s, ts on right and s, st on the left. Not the same cosets. So I, s, s2 is a normal subgroup of Dih3, with a coset of t, st, s2t, while I, t (or I, l and I, r) isn't a normal subgroup.
We could also have looked at conjugation, e.g. tIt=I, tst=s2, ts2t=s, and this is true for conjugation by any element in Dih3.