What does simmer slowly mean with white soybeans? The instructions in the soybeans say that parboil first and then simmer slowly for two hours.

Let's consider this in the scale of my plate. The power-scale of my plate is from 1 to 6. I put it first to 2, then to 3 and then to 4. It started to boil little with 4. Therefore, I consider that simmer slowly means no bubbles in the water i.e. 3.

I assume that parboil means "put the power of the plate to 6 and wait that everything bubbles vigorously".
I explain what the technical definition between a simmer, boil and roiling boil is here.  A slow simmer would be a very thin but continuous stream of bubbles rising from the bottom of the pot. 

Parboiling does not mean maximum heat. Parboiling means that you're only boiling until it's part of the way cooked, not completely cooked; so, you're cooking mostly at a full boil, then reducing heat to a simmer to finish them.  This ensures they don't overcook.

You'll have to figure what setting a boil and a simmer are on your plate, as it depends on the power and size of your pot.   On a stove, I'd expect 2 to be a simmer, and about 3 as a full boil, but it varies from one model to another.