The just-world hypothesis is a belief some people have that the world is fair and just. They think that good things happen to good people, and bad things happen to bad people. Essentially, they believe that everything happens for a reason and everyone gets what they deserve.
It's kind of like if you do your homework and study really hard, you'll do well on your test. But if you don't do your homework and don't study, you might do poorly on your test. The just-world hypothesis takes this idea to the extreme and applies it to everything in life.
But the problem with this belief is that it's not always true. Sometimes, bad things happen to good people, and good things happen to bad people. For example, someone might get sick even though they've always been really healthy and taken good care of themselves. Or someone might do something really mean but not get caught or punished for it.
So while it's good to believe that we should all try to do our best and be good people, we also have to remember that sometimes things happen that are out of our control. We have to be kind and understanding towards ourselves and others, even when things don't seem fair.