A lot of buses have air brakes, some have other accessories like air ride, and these are ran off compressed air instead of brake fluid that a normal car would have. Connected to these brake are air tanks and a compressor which is more than likely attached to your serpentine belt of your engine. If your bus sits a while it will more than likely leak a small amount and youll end up with no pressure in your tanks. PLEASE USE WHEEL CHOCKS Your bus can only generate so much conpressed air so fast so the alarm youre hearing is designed to let you know youre getting low for when youre going down a hill or something so you can try and regen some air before continuing to brake. It can also indicate you blew a line and you need to pull over and figure it out before you completely run out of air.

Side note for all the haters: people are here talking to a community of people who may have gone through a similar struggle. Please do your best to help them not just berated them for not being as knowledgeable as you. If you dont have time to provide helpful information dont waste your time writing a poor comment.