Just like every body of water, over time the water will begin to evaporate. Typically you can expect about a quarter inch of water or less to evaporate each day in an uncovered pool. There are a number of things that contribute to this. Wind, sunlight, and humidity are the key players in water evaporation.
Wind can blow across the surface of your pool which will cause the water molecules to evaporate much faster.
Direct, intense sunlight will help to dry up water molecules and cause more rapid evaporation.
Humidity in the air plays a big role in evaporation. The more humid the air is, the less water in your pool will evaporate. Conversely, during dry spells you can expect more water loss through evaporation. If you think about the air outside like a sponge then you can expect that if it is dry, then it will be able to pick up more water from your pool, whereas if it is already wet then it will be limited in how much water it can soak up.
So how do you know if you have a leak or if the water is just evaporating?
You can not be certain just by measuring the amount of water loss in you pool per day. However you can conduct what is know as the bucket test. To do this it is best to start with a full pool. Then fill a five gallon bucket 2/3 of the way full and set it on the second step of the pool. From there you will mark the water level on the inside of the bucket. Next turn off you pump and mark the water level on the outside of the bucket. Once that is done, turn the pump on again and let it sit for at least 24 hours. At that point you can compare the water levels. If the level on the outside of the bucket has gone down further than on the inside then there is likely to be a leak in your pool. If the level is the same both inside the bucket and outside then the water loss is likely to be due to evaporation.