I didn't understand this sentence

What does “that much” mean in this sentence:

That much I understand.

I didn’t understand.=/

Whatever it was that the other person was explaining, this person is saying “That much I understand” = “I understood THAT part of the explanation already” = “THAT part of it was clear to me before”

it’s just an inversion. The same as
I understand that much.

Thanks for the answers.

In fact, I didn’t know the meaning of “much” in that sentence.

Why in the first sentence, the verb " find " is not used to she.
EX: My brother’s girlfriend… ( finds) and no find.
Does anyeone help me?

The sentence is “I want her to find…”

Jane wants her (her brother’s girlfriend) to come home and find Sally in their bed.