What is an idea or explanation that leads to scientific investigation called?

A hypothesis, in essence, is some random thing some random person comes up with, and using the scientific method, carries out experiments to test the validity of their hypothesis. If a hypothesis is tested valid, it's either a result of existing physical laws, or if not, sometimes a law or theory can be improved by results from the experiments.

Once a person tests their hypothesis and finds it to be valid, they can then develop a theory based on their data conclusions and results. Once a theory becomes readily accepted from the scientific community, it can become a law or a principle. Some theories are both true and false, but still not be invalidated. For example, Newton's theory that all objects fall at the same velocity wasn't wrong; it was just incomplete; i.e. it had a range of validity. Air resistance and terminal speeds, as well as relativistic effects (which Einstein tested) provide a more complete view on the effect of gravity on objects.

A theory is an explanation for why something happens, and if the explanation is sound, the theory should be able to accurately predict future events.

The prediction is known as a hypothesis, which is a statement about what is expected to occur in the experiment. If the hypothesis is correct, it supports the theory; if it is incorrect, it casts doubt on the theory.

Based on his theory of relativity, Einstein predicted—and even calculated the degree of bending—that light would be bent by a gravitational field. According to his theory, gravity would cause the light to bend because it would slow down the time it experiences.

The concept that time was relative and subject to change seemed absurd to people in his day. When the prediction was put to the test during a solar eclipse, people were shocked to learn that Einstein was right, and since his theory's underlying hypothesis was supported by experimental data, his theory was accepted. (Note that many scientists, even in this day and age, still do not consider the possibility that time is not constant in the universe.)

Newton's laws of gravity were not contradicted by Einstein's theory of gravity. Theories explain why certain things happen, while laws describe what will (always) happen. Theories are tested by hypotheses, which forecast the results of experiments.

Hypothesis.