Lockers mechanically lock both tires on the axle together, and then to the driveshaft. An open differential, such as you have now, will allow 1 tire on the axle spin slower than the driveline, which is good for on-road cornering (the inside tire has less distance to travel than the outside tire and will turn slightly slower). However, that also means that 1 tire can be stopped while the other tire spins away, which is not good for off-road.
There are 3 ways to lock your axle:
Spool: A spool is a device which deletes all of the open diff's spider and side gears (these gears allow tire speed differentiation). A spool basically bolts the driver and passenger side axle shafts together and then to the driveshaft. When cornering, the inside tires will spin out slightly as the outside tire skids lightly.
Automatic: This locker will allow a tire to spin faster than the driveline, but not slower. Basically it functions as a spool until a corner, the outside tire will "unlock" and over run the inside tire. This technically makes only 1 tire recieving all the power until the vehicle straightens back out, or power overcomes traction on the inside tire and the inside tire will break loose and speed up to the outside tires speed. Or much more power is applied and both tires can break loose. Depending on traction/terrain, this could require a lot more power or a little.
The Auto locker has 2 sub categories: Full Case/Carrier or "Drop-In/Lunchbox". The Full Case, such as a Detroit Locker, replaces the OE case/carrier and can be much stronger. The "Drop-In" or "Lunchbox Locker", such as a Lock-Right or Aussie Locker, goes inside the OE case/carrier (both case and carrier refer to the same thing) and replaces the spider gears and side gears, which are the gears that allow an open diff to let a wheel stop while the other spins.
Selectible: The axle can be manually locked or unlocked via compressed air, cables, electricity. (ARB, OX, Auburn ECTED, etc.) When open, the axle will perform as an open differential. Once locked, the axle will act like a "spool".
Many folks have different opinions on which is the best/which they prefer. For a rear application, I prefer a Selectible or Full-Case Auto locker, since they are stronger and seem to last longer than the Drop-In lockers. A locked axle can see much more stress than an open differential.