'Pop out' is a phrasal verb meaning to 'go out for a short time'. "I'm just going to pop out for some fresh air." If you say something suddenly without thinking about it first you might say 'the words popped out'. "Sorry, I didn't phrase that very well, the words just popped out." To 'pop out' of somewhere can also mean to appear suddenly. "I got a shock when I was driving and a deer popped out onto the road." If someone has a big reaction to something or can't believe what they are seeing you can say their eyes 'popped out' of their head. "His eyes popped out of his head when he saw the enormous birthday cake they had bought for him." To 'pop out' can also mean to 'come out'. "If you put in your details the results will pop out here." 'Pop out' is quite a casual phrasal verb used more in social contexts.