Between me, chip, and gareth, its probably been explained a 50 times on here keith

Very basically...

Being below the Boost Threshold is the RPM range where there is simply not enough exhaust gas to spin up the turbo.
(Ie when you boot your T4'd cossie engine at 2000rpm and fuck all happens)

Lag is the time it takes from foot mashed to the floor to when you got noticable boost, when you IN the boost threshold.
(Ie the very slight delay between mashing the throttle and decent acceleration when you boot a T4'd cossie engine at 6500rpm)