Real simply put a brown out is when your receiver battery voltage drops below around 4 volts. Never use anything less than a 6 volt battery in your receiver. Always use a quality charger and check battery voltage prior to each flight. If using digital servos be expecially diligent with your onboard battery. If you are performing 3D it will suck voltage quickly and can cause brownouts and blackouts if your battery is inadeqate.

You can place a voltwatch on your plane and with the equipment turned on. Place all your controls in full travel and look at your voltwatch.

THis will tell you the story