The Big Five personality traits are five broad areas of personality that are used by psychologists when looking at....personality ;). The Big Five factors are Openness, Conscientiousness, Extraversion, Agreeableness, and Neuroticism.
AKA- five really cool ways to look at the micro-characterization of your characters!
The idea behind personality theory is that certain traits indicate specific personality types, and for writers, if we understand what type of personality our characters have, our development of them will be much more rich and real. Little subtle cues will add depth- your reader may not realize where it's coming from, but they will know it's there.
Understanding aspects of the human personality can really help us as writers build stronger and more complex characters (and yes, there are a lot of different personality theories out there ;))
There are tons of huge books and articles on personality theory and the Big Five, but unless you are really interested as a source of study outside of your writing, you can probably just find some easy short online tests. I have no idea of the validity nor reliability of these tests-LOL. The idea is just for you to get a feel for the scale of personality traits within the big five and see how they could be applied to your writing (who knows, maybe you'll find out something about yourself as well!