Question

heat is the transfer of thermal energy because of a difference in what

Answer 1

Heat "TRANSFER" is what we're talking about here.
Not just heat.

And it looks like it's talking about a difference in "temperature."

Think about this.

When you put a pot of cool water on a hot stove,
there is a difference in temperature, between the water, and the stove.

That's why heat goes from the stove, to the water.

It's because of that difference in temperature.