A universal is something that is true of anything’s nature. For instance, a human is a rational-animal. That means, he can think in abstracts (he can think of “redness,” he can do things a computer or animal cannot do with his mind) and he is also physical (he has a material body, like other animals). So the thing that ties all humans together is that they’re rational-animals.
A particular is something that is particular to a nature, but not in the definition of the nature (a property). Then there are things that flow from the particulars that are called “accidents.” So let’s take John.
Universal – John is a rational animal
Particular – John can run
Accident – John can run faster than most men
If we look at David, we can see the following:
Universal – David is a rational human being
Particular – David cannot run
Accident – David must be in a wheelchair because he cannot use his legs
Now, while David might have the capacity to run (if his legs worked), he currently cannot. But when the universal is in the right place, he’s still a human being even if he doesn’t share in all the properties.