What does 750 on gold ring mean?

Karat - The karat (symbol kt) a unit of measure of the amount of pure gold in a metal. One karat represents 1/24th of an alloy's total weight. One hundred percent gold is 24 karats.

24-karat gold = .995 to .99999

22-karat gold = .916

18-karat gold = .750

14-karat gold = .583

10-karat gold = .4167