In the main phase of any star's life, hydrogen is being fused into helium inside the core. This phase takes up 90% of the star's life span, and this is the phase the sun is at now. The next phase is where it turns into a red giant. Once the hydrogen has completely turned into helium, the inert helium ash collapses on itself. Which in turn causes the core to get denser and heat up, which also expands its size. It appears red because outer surface is a lower temperature, that makes it appear red (hence the name "Red Giant")
