An apple fell.
Gravity was discovered when an Issaac Newton had an apple fall on his head.
How no one had discovered this force before 1665 is unknown.
Gravity affects your weight. Your weight is defined as the effect of gravity on your mass. Mass cannot be created or destoryed so if you went to a plent with less gravity you would weigh less but your mass would remain the same.
All objects exert gravity on all other objects. But because gravity is the weakest of all the fundamental forces of the universe its effects are not noticed until things reach a massive scale. We all exert forces of gravity on all of the things around us but because we are so small on a comsmological scale we do not see these effects in our day to day lives.
Gravity is the weakest findamental force in the universe but it is also what helps shape the flow of the space time continuum.