I always here this term Without Loss of Generality in my math courses at university? What does it mean?
You normally hear about the term Without Loss of Generality when you're trying prove something in math. It means the example you are using to proof the solution will work in other case.
If you had a bag of marbles (3 or more marbles) that were either green or orange, then at least two have the same color.
Assume Without Loss of Generality that the first marble you pull is green. If the second marble is green, you have two green marbles, otherwise you have two orange marbles.
You can use WLOG here because the proof is very similar to if you pulled an orange one first.