The sun is currently enjoying the middle portion of its life cycle but as it reaches the end of its cycle (in an estimated 4-5 billion years) it sun will expand into a red giant about the size of the earth’s entire orbit. This expansion will almost certainly engulf the Earth, or at the very least scorch our planet beyond recognition.
Our fate however, is decided much sooner than that. As the sun slowly begins to expand over the course of millions of years, the temperature of our planet will increase until it is no longer capable of supporting life. Scientists believe that could occur as early as 1 billion years from now - much sooner than the estimated 4-5 billion years it will take for the sun to expand far enough to swallow our planet.
If mankind can survive another billion years on this rock they will need to prepare for life elsewhere as the increasing temperatures evaporate all of Earth’s rivers, lakes, and oceans and cause a greenhouse effect that only the hardiest of microorganisms will be able to survive.