“my covenant is with you.” This is idiomatic for, “I have made my covenant with you.” This is a clarification and expansion of Genesis 15:18, and an expansion of what God said in Genesis 12:3, “I will make of you a great nation.”
“multitude.” The Hebrew word is hamōn (#01995 הָמוֹן) and it means “murmur, roar, rush, tumult, confusion, crowd, multitude.” It often refers more to the sound a crowd or multitude makes than the number of the crowd, although a large number is certainly implied. This is a good description of the multitude of nations that would come from Abraham, and a very accurate prophecy. Far from being friends and similar in culture and custom, Abraham’s descendants are very different and sadly, often even enemies.
“nations.” Genesis 17:4, “You will be the father of a multitude of nations.” is quoted in Romans 4:17, where it includes the Gentile nations who trust in God (Rom. 4:16)