Every individual has a different physical features and personality traits. That is why we are unique. Even identical twins have differences.
Traits in The Sims Mobile are distinguishing qualities of a certain Sim. With the latest 2.2.4 update, Sim's trait was given improvements. Doing certain interaction that matches the trait of a Sim can give you bonuses. Some traits improve bonus and XP you'll receive. Each trait is different to each other. By default, you have one trait per Sim. As you play the game, you can unlock some other slot for another trait.
List of Traits
Generous These Sims give a bit better daily Login Rewards
Ambitious These Sims are a bit better at all careers
Foodie These Sims are better at the Barista and Culinary Careers
Lucky These Sims occasionally give you bonus XP while at home
Flirty These Sims get a bonus every now and then when being flirty with others
Good These Sims occasionally get a bonus when being friendly with others
Mean These Sims get a bonus every now and then when being insulting with others
Pickpocket These Sims will sneak a few Simoleons from others, every now and then.
Creative These Sims are a better at the Fashion Career
Musical These Sims are a bit better at Piano and Guitar
Charismatic These Sims are a bit better at Business and Law Careers
Talented These Sims are a bit better at all Hobbies
Kind of a Big Deal These Sims are better at the DJ Career
Active These Sims are a bit better at Yoga
Party Animal These Sims get bonus Party XP every now and then when at parties
Alluring These Sims are a bit better at Romantic Events
Outgoing These Sims are a bit better at Friendly Events
Competitive These Sims are a bit better at Rival Events
Genius These Sims are a bit better at the Medical Career
Artisan These Sims are a bit better at Writing and Cooking
Good Upbringing These Sims are way better at all Events