A hernia is caused when the lining around your internal organs tears and something pops through it. (often it's the intestines that come through). It's nothing to panic over; he's not in danger, but he needs to be given ( and forced to take) the time to heal himself so it doesn't get worse. Really little hernias can turn into bigger ones, and those are harder to heal.
It's most commonly caused by lifting up something too heavy.
It can be very painful, and he'll have to be careful not to make it worse, by continuing lifting things fir example, while it heals.
His doctor has probably limited his activity level for a while to see if it heals on it's own. If it doesn't, it can be surgically repaired.