if you are a noob, then i would suggest sticking with GPS mode until you are comfortable with controlling the craft. in GPS mode, the craft will try to stay in the last known position that it received input from you. meaning that if you were flying and then let go of the control sticks the craft will stay put (no sliding since it is trying to stay at the same location). you also really shouldn't be messing with any of the settings in the NAZA software yet, with the exception of calibrating the TX & IMU.
GPS signal loss does not make the craft crash. however you do have to be vigilant because if the craft looses GPS signal in flight and then regains it, it will record the spot where it regained GPS signal as the "home" position. this also includes the altitude that it was at when it re-established GPS signal. if this was to happen and you had to do a fail safe return to home command, your craft will fly back to the new spot and land there. meaning that if it was 60 feet up in the air over the roof of some building, that is where it will try to land and fall the rest of the way onto that roof.
in ATTI mode, the craft maintains a level flight, however when you let go of the control stick the craft will continue to fly in a straight line in the direction it was heading until something stops it, you interact and give it new command, or eventually air friction will slow it down. primary difference between GPS and ATTI mode is the immediate hover in GPS instead of the "slide" that happens with ATTI.
hopes this helps