A brown out is defined as the loss of signal caused by to low a voltage on the receiver buss. This varies depending on the receiver design but is usually about 3 volts. This can be cause by several factors some of which are 1; to small a battery in so far as mah's go, to high an internal resistance of the battery, 2: high impedance between the battery terminals and the receiver buss (such as to small a gage wire or bad switch) 3: insufficient capacity from an ESC's BEC, 4: stalled servo or to much current demand from any device getting commands from the receiver, 5; many other possible causes. In most cases this can be prevented by just having an adequate power source for the receiver with good connections.