Here's why:

* Stars evolve: Stars go through different stages in their lives, and red giants are one of these stages.

* Characteristics of red giants: Red giants are stars that have exhausted the hydrogen fuel in their core and have expanded significantly. They are cooler than the main sequence stars they were before, which is why they appear red.

So, a red giant is a star that has evolved to a specific stage in its lifetime, characterized by its red color and large size.