“Now a mediator is not of one, but God is one.” ( Galatians 3:20, King James Version )

I have not been to church in years. I recently went to a friend’s prayer meeting. While there, I was encouraged to read the following from the bible:

"Now a mediator is not of one, but God is one." ( Galatians 3:20, King James Version )

The number one reason I have not been to church in years is that when I did go to church, I had no idea what people were talking about. They might as well be speaking Swahili, Mandarin, or Greek for one hour of time.

Please, answer this question so that an ignoramus, or a child, might understand.

What does the following mean in 21st-century American English to someone who has never read the Bible before:

"Now a mediator is not of one, but God is one" Galatians 3:20, King James Version

I think that the New Living Translation might be easier for people like me to understand:

Galatians 3:20, NLT: "Now a mediator is helpful if more than one party must reach an agreement. But God, who is one, did not use a mediator when he gave his promise to Abraham."

What promise was given to Abraham? What is the minimum amount of context required to understand Galatians 3:20?