Notice that the heavy chain is in blue, the light chain in green and the protein being bound is in red. Your hands each represent a binding place - but your hands can bind only to one particular shape. If these circulating antibodies come in contact with the thing they were generated to appear against in the first place, then, the antibodies will bind to the foreign thing - the target. Therefore, after awhile, a toxin, a virus, or a bacterium will no longer to be able to hurt us. Therefore, we will make antibodies which can bind to the thing we are injected with.
Antibodies are also very important tools used in medicine and science.
All in all then, antibody molecules are really nice proteins - just one of the many useful things our immune system does for us. You know, sometimes I just cannot believe how elegant all of these things are - amazing. We are very, very lucky.