Hi Kevin,

The shortest explanation I can give is that in the sunspot umbra the magnetic field stops energy from getting to the sun's surface, but in the penumbra the magnetic field is different and this does not happen as much.

To understand why sunspots are dark it helps to understand why the sun is bright. Hot, material rises from inside the sun, spreads out at the surface, releases energy in the form of light, cools and sinks back down.

However, in sunspots this process does not work as well as in other places on the Sun.
This is especially true in the umbra where the magnetic field is strongest and is directed straight up and down. In the penumbra the field is more complicated and more pointed sideways rather that up and down. The magnetic field in the penumbra does not interfere with the convection as much as in the umbra, so it is brighter there.

I know that is a kind of complicated explanation, but I am not sure how to simplify it more.
Here is a picture of a sunspot.
https://solarscience.msfc.nasa.gov/images/sunspot1.jpg
You can see the places where convection is happening in all those little shapes on the surface of the sun around the sunspots. They are called "granules"
https://solarscience.msfc.nasa.gov/images/granules.jpg

Hope that gives you some idea,
Terry