Well I don't know if it is the same for Jass but I can tell you what it is in languages like C, C#...
Object Oriented Programming is a way to structure your programs.
An example:
If you would have to program a lamp, you would program:
The bulb
The socket
...
And everything would be single Object.
Now the bulb for example would be able to repair itself.
So you split everything up into objects which control themselves.
Atm I am not sure whether I explained that complitly right but you can check wikipedia...