What is Brake Hold in my new Toyota? 

You press it every time.

I don't believe the feature is designed to be active on every trip. It's designed to be used in line at the drive through and then disabled when you resume driving.

Good point , the Audi system always stays on.

Yes, even though they are electronically applied, the brake lights come on. Same with the dynamic radar cruise control. When it slows you down, the brake lights are activated.

Press "Hold" to disengage the braking system

With your foot on the foot brake press the hold button just once to cancel. See p214 of the RAV4 free Manual, which you can download as a pdf. It says ‘To turn the system off while the system is holding the brake, firmly depress the brake pedal and press the hold button again.

How? It's so you don't always have to keep your foot on the brake when at a red light or in traffic lol.

That system is not designed to be on at all times.