Personality theory is one of the core concepts in psychology. Freud made a big contribution to its development, but he isn't really considered a theory himself. Other people had different ideas about personality and their thoughts also helped to develop what we call the "Big Five". Try thinking of a person you know pretty well, and ask yourself where they fit on each of these 5 points:
1. Extroversion vs. Introversion (or outgoing vs. shy)
2. Neuroticism vs. Anxious vs. Relaxed (perfectionist/ control-freak; worry-wart or mellow?)
3. Open-ness vs. Closed to Experience (Eager to try new things or Prefers Predictable/ conventional routine)
4. Agreeableness vs. Antagonism (A peace-maker or a fight-picker?)
5. Conscientiousness vs. Undirectedness (Hard-working, goal oriented or Lazy?)
These five points can mix and match to form personality. People like Carl Jung and Carl Rogers did a lot of work with personality theories. If you've ever taken an internet quiz that asks something like "What kind of person are you?" then you probably answered questions related to the "Big Five"
