A statement that might be true, which can then be tested.
Example: the hypothesis that larger dogs are better at catching balls was about to be tested with 100s of different sized dogs.
Sometimes the hypothesis won't be tested, it is simply a good explanation (which could be wrong).
Example: you notice the temperature drops just as the sun rises. Your hypothesis is that the sun warms the air high above you, which rises and so cooler air come from the sides.