Chanley- All it means is that urine fills the space between the foreskin and the glans before going out the hole at the tip of the foreskin. The foreskin puffs out a little
If a man has phimosis, the balooning would be one of the symptoms of having a very tight opening to the foreskin,
but young boys who have snug foreskin openings do no have phimosis so their balloning is not an indicator of a problem... rather it is a outward visual clue that his mom can see that the sepration process between the foreskin and glans is done/almost done without actually retracting him (if his foreskin opening is still snug -that could really hurt him to try to retract and see if there were any adhesions thee) So his mom is doing a good job of just sitting back and obsrving outward signs that things are moving along without prying.
I have heard stories of schoolboys who were able to do balooning games (you know- a pising contest) but beyond that- I do not know the object.
My son (he's5) somtimes the opening of his foreskin and his meatus are not alligned because of how his underpants were smushing- since I have never instructed him to retract to pee- so when he starts to pee it might baloon for a second until the pressure finds the exit and things get lined up.