Please tell me what's the difference between free testosterone and total testosterone?

Think HS prom nite. Most Testosterone is bound up by Sex Hormone Binding Globulin (SHBG) and is unavailable to do what we associated w/T. Only a small fraction of Total T is available, what we measure/calculate as Free T. Think of Total T as all the girls at your high school's formal dance. Most attended w/their date (represented by SHBG). You're more interested in those girls who attended by themselves eg Free T. .