To answer that, we first have to understand how our bodies are put together. Picture a hot dog or sausage. If you've ever seen them get made, you'll know that the meat for them gets put into a casing. The casing helps the hot dog or sausage keep the meat together and give it its shape.
What the casing does for the sausage or hot dog, our body does for us. But unlike hot dogs, humans are vastly more complex, so we tend to have a number of different types of layers keeping our bits in place. The idea, however, is the same.
Now picture a sausage in its casing, then see a pin and watch it make a small hole or tear in it. If you move the sausage around enough, some of the meat inside the casing will wiggle out. That bit of meat or fat that has escaped through the tear or hole in the casing is the hernia.
It's not easy to make out, but the picture above shows a hernia that has formed in the lower right quadrant of the abdomen due to a weakened muscle wall due to a previous surgery. It's not good to leave a hernia like that continue to grow or sit there as they can cause pain, and worse, if it twists and the blood flow to the escaping bits get cut off, it will turn rotten and cause all sorts of problems.
Nowadays, fixing hernias is a lot easier than it used to be. For the problem pictured above, all it took was an out patient laparoscopic surgery. Putting in a scope through a cut in the bellybutton, they can see the exact problem area (and take pics!). With two other small incisions, they push the escaping bits back where they belong then drag in a mesh, which gets stapled to the surrounding area like a patch, to seal it up and put a stop to any further escape attempts by your innards.
Science is awesome!
(And yes, these are MY insides! Us writerly types must use all that is presented to us for writing fodder. Yes, yes, we must.) Heh heh.