Your pool will naturally lose some water to evaporation, some to splash-out and some to backwash wastewater. You may also gain water from rainfall. My rule of thumb is that if you're routinely adding more than two inches of water to your pool per week, you may have a leak. It is worth spending some time and money to repair.
Pools are meant to be watertight but sealants will deteriorate while other parts of your pool shift and settle or wear out. Pools can leak through any of the fittings or accessories, plumbing, or even right through the shell. It is important to repair leaks, not only to save water, heat, and chemicals, but also to prevent undermining pool structural components and washing away fill dirt.
Leak detection is a highly specialized branch of the industry. Ninety five percent of all phone calls I get from worried pool owners about a leak turn out to be inexpensive to repair.
If you do suspect a leak, one common cause is the skimmer needs replacing.
If your plastic skimmer has pulled away from the concrete pool and is leaking, use an underwater pool putty to patch it up temporarily and stop the leak. This is a very common pool leak source. You may want to use a dye test to determine if your skimmers are leaking. Small debris stuck in a small crack is also a clue of a leak. Skimmers can also get cracks in the plastic from concrete expansion/heave. Again pool putty is used to repair this.
If your skimmer weir (the flapper gate) has come out or broken, replace it with a new one. The weir creates a small waterfall into the basket which speeds up water flow, drawing more debris in. The weir also helps to keep debris in the skimmer neck when the pump shuts off. Similarly, replace a broken skimmer lid , before someone steps in it.
If you suspect that your skimmer line may be clogged, use a plumber's snake to try and break up the leaves & sticks (or whatever), or better yet, try a "drain king " which attaches to the garden hose and puts high pressure in the line. Try it in both directions, that is, from skimmer towards pump, and from pump towards skimmer. I usually use a plug at the skimmer end to build up pressure in the line for 5-10 seconds. Repeatedly doing this quickly is the best way to clear a pipe.