Shower resealing is one way to waterproof the shower and prevent future leaks. If you have a shower, then it probably has a liner. Most shower liners are plastic that is glued on the top of the shower pan, with a piece of olefin or some other glue on the bottom. If the shower leaks, then the glue will get stuck to the shower floor and seal the leak. When this happens, you can either make a trip to the home improvement store and buy a special sealer that is supposed to fix this type of problem, or you can try some more flexible ways.
One of the easiest and cheapest ways that I know of for shower resealing is using some inexpensive silicone caulk that you can purchase at any hardware store. Some of these will be flexible, but they usually also have an adhesive side that means that you can either glue them to your bathroom floor or your walls or whatever else you want to adhere them to. The advantage to these kinds of products is that they are relatively cheap and easy to apply, as well as being flexible and usable in a wide variety of flooring types and surfaces. One downside to this kind of products is that if you happen to use masonry tile, then you will probably have to remove all the tiles from the floor before you can apply the caulk.
If your shower is leaking is caused by masonry tiles or other slabs, then one of the most flexible ways to go about solving the leak is to use a masonry sealant. I recommend going to a home improvement store for these kinds of sealants because there are many different ones out there, and some may be more suited to your needs than others. For example, one of the cheaper sealants that you can buy is some kind of liquid silicon based sealant that is meant to be poured into the joints between the tiles. Another similar product that you can buy for shower resealing is some kind of liquid rubber sealant that you can spray on the tiles after they have been fixed into place. Usually these two kinds of products are more expensive but are both effective at stopping leaks and preventing further leaks from occurring.