In essence, faith is trusting someone or something.
In Gen. 15 God tells Abram that he will give him a child and that Abram’s descendants will grow in number to become a nation:
…the word of the Lord came to him: “…your very own son shall be your heir.” And he brought him outside and said, “Look toward heaven, and number the stars, if you are able to number them.” Then he said to him, “So shall your offspring be.” (Gen. 15:4-5)
Abram’s response is recorded in the next verse:
he believed the Lord… (Gen. 15:6)
Abram trusted God to do what he said. And, because God is faithful, he kept his promise to Abram and gave him a son (Gen. 21:1-2), and his descendants became a nation, the nation of Israel.
God is faithful, so we can trust him, we can put our faith in him.