A hypothesis is a proposed explanation for an observation. It is an educated guess about how things might work and can be proven or disproved through testing and experimentation.
When you are faced with a problem, you come up with a hypothesis to solve it. Your hypothesis can only be proven or disproven by testing it through experimentation or testing.
The theory of gravity is a hypothesis. When Newton wrote his book, he used mathematics and observation to show that what had been thought of as the phenomena caused by a force were actually explained by mathematical concepts.