The document discusses dihedral groups Dn, which are the symmetry groups of regular polygons with n sides. It specifically examines D4, the symmetry group of a square with 8 elements - the 4 rotations (r0, r1, r2, r3) and 4 reflections (s0, s1, s2, s3). It then shows that D4 forms a group by verifying the group properties of closure, associativity, identity, and inverse elements. The document will next explore the symmetry group D5 of a pentagon.