Free testosterone is the unbound, biologically active fraction of total testosterone, typically only 2-3% of the total. It's the form that actually binds to androgen receptors and drives the effects you feel: energy, libido, muscle growth, and mood. Men with high SHBG may have normal total T but low free T, experiencing symptoms despite 'normal' labs.
Equilibrium dialysis or calculated free T (from total T, SHBG, and albumin) are more accurate than direct immunoassay methods.