Back wheel is "wobbling"
There are a few things that could be described as a 'wobbly' wheel. 

Hold the rear wheel off the ground, grab the tire at the 12 o'clock and 9 o'clock (from the chain side of the bike), try to move the rim sideways. If the wheel feels like it is moving (wobbling) on the axle (other that rotating), the hub bearings are damaged or need adjusting. Your local bike repair store can tell you if they can repair the hub, or if a new wheel is required.

Hold the rear wheel off the ground, spin the wheel, watch the rim relative to the brake pads. If the rim looks like it is moving in and out relative to the pads (wobbling) as it spins, the rim is out of true. If it moves a few millimeters, it wont really affect the bike. Any more, or if the rim is hitting the brake pads, the rim needs to be trued. Again, your local bike repair store can fix that for you.

Lastly, the tire may have become uncentered on the rim. If the rim runs true but the tire does not, you can deflate the tire and push and pull it to re-seat the bead on the rim.