Genesis 17:3-5 meaning

Abram fell on his face in awe, humbling himself in front of God in submission. Falling on one's face was a way of showing respect to one's superior, or suzerain, and of assuming the role of a servant or vassal (Genesis 37:10, 42:6, 44:14, 48:12; 2 Samuel 9:6; 1 Kings 18:7). 

God now further expands His covenant commitment to Abram (Genesis 15:8-21). God had already promised that Abram would have a large number of descendants (Genesis 15:5). God now adds You will be the father of a multitude of nations. God grants an additional reward for faithful service. Abram will be the father of a multitude of nations. God had already promised that Abraham would have a large number of descendants. God grants this reward after making a mutual agreement with Abram that he will be made great if he walks in righteousness. Now He promises that he will be the father of a number of nations. 

In the present time, this promise has already been fulfilled in two ways. First,  the people physically descended from Abraham constitute many nations (Genesis 25-26). However, Abraham is also a spiritual father to all who believe  (Romans 4:9-11). This would include people from every nation (Rev 7:9). God has, indeed, fulfilled this promise granted to Abram to be the father of many nations. God also rewarded Abram pursuant to the conditional promise to multiply Abram exceedingly if Abram walked before God blameless because, as we will see, Abram fulfilled his part of the agreement.

In ancient times people attached great significance to the names of people and places. One's name often said something about their origins, character or destiny. The one who gives a name has power over the one named. This is the first time that God changes an individual's name. Your name shall be Abraham. Abram's name (exalted father) was changed to Abraham (father of many) because he would become the father of a multitude of nations. His new name recognizes him as the father of the faithful, regardless of ethnic race. By repeating this promise again, I will make you the father of a multitude of nations, its importance is reiterated and highlighted.