"Constituent" is just another way of saying "a part." Like Thor is a constituent of the Avengers. Or like your hand is a constituent of your body. Or like your mom is a member of the Disappointed Mothers of America.
The term frequently comes up in finance in reference to stock indices, like the Dow Jones Industrial Average or the S&P 500. In this case, the word refers to one of the stocks belonging to that index...like in the sentence: "Apple is a constituent of both the Dow and the S&P 500."
Constituent also has meaning in politics. It refers to the people a lawmaker represents. As in the sentence: "My constituents won't like that I voted this way, but this briefcase full of money makes the hassle well worth it."