Inside a star two effects compete. Gravity pulls all the material inwards and tries to squash the star. Nuclear fusion in the core releases energy and makes the gas extremely hot. Hot gas expands, and this thermal expansion pushes outwards.
During the main sequence these two effects balance: the inward pull of gravity equals the outward push of thermal expansion. The star stays the same size and is stable. This lasts as long as there is enough hydrogen to fuse.
When the hydrogen in the core runs low, fusion slows down and gravity wins at first, so the core contracts and heats up. This heating makes the outer layers expand and the star becomes a red giant. A star like the Sun finally runs out of fuel and gravity squeezes the core into a white dwarf. In a much more massive star, the core fuses heavier and heavier elements until it cannot fuse any more. Then gravity makes the core collapse suddenly, causing a supernova.