Stars can go through two red giant phases. After stars burn up hydrogen and become red giants, the core may shrink and allow helium to be used for fuel, returning them to main sequence stage. When the new fuel is burned up, the star may return to a red giant.
Red giants are massive stars, transforming to become up to 400 times larger than their size in the main sequence phase. The red color is due to cooler temperatures. Red giants are short lived and cool to blackness as they become degenerate dwarf or neutron stars. Some collapse to black holes. Betelgeuse is an example of a red star in the Orion constellation.