A simple picture
Imagine a star that burns fuel very fast. It shines brightly, grows into a giant stage, and changes color depending on how hot its surface is.
Red supergiants are cooler on the outside and very wide. Blue supergiants are hotter and shine blue-white.
Could Earth orbit inside one?
If some red supergiants were placed where the Sun is, their outer layers could reach past Mars. That does not mean planets could safely live there; it is only a size comparison.
Space scale is strange, so comparisons help.
Why scientists study them
Supergiants make and spread heavy elements. When some explode, they send material into space that can later become new stars, planets, and life-friendly chemistry.