Abram bowed low with his face to the ground
"Abram threw himself face down on the ground" or "Abraham immediately lay down with his face to the ground." He did this to show that he respected God and would obey him.
As for me
God used this phrase to introduce what he would do for Abram as part of his covenant with Abram.
behold, my covenant is with you
The word "behold" here says that what comes next is certain: "my covenant is certainly with you."
the father of a multitude of nations
"the father of a great number of nations" or "the one after whom many nations name themselves"
Translators may add the following footnote: The name "Abram" means "exalted father" and the name "Abraham" sounds like "father of a multitude."
I will make you exceedingly fruitful
"I will cause you to have very many descendants"
I will make nations of you
"I will cause your descendants to become nations"
kings will descend from you
"among your descendants there will be kings" or "some of your descendants will be kings"