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. 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.
Filling all of your Sim's trait slots will give you a prompt to retire a Sim. If you missed or disregarded the retirement notification, you have to imaginary raise another trait to get the prompt or the option to retire. The game doesn't have a progress bar for traits. Basically, you need to play and pray to receive that option again.
To raise another trait, consider the traits that you have with your Sim. For an example, if you have "Kind of a Big Deal" trait that boost DJ Career, assign your Sim to be a DJ and perform lots of DJ career events.
List of Traits
Generous These Sims give a bit better daily Login Rewards. Unlock Lv 2/3 using Poppy's Generosity Charm
Ambitious These Sims are a bit better at all careers. Unlock Lv 2/3 using Gabby's Ambition Charm
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. Unlock Lv 2/3 using Chaz's Flirty Charm
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. Unlock only using Carl's Mean Charm
Pickpocket These Sims will sneak a few Simoleons from others, every now and then.
Creative These Sims are a better at the Fashion Career. Unlock only using Violet's Creativity Charm
Musical These Sims are a bit better at Piano and Guitar. Unlock only using DJ Candy's Rhythm Charm
Charismatic These Sims are a bit better at Business and Law Careers. Unlock only using T.O.B.O.R.'s Charisma Charm
Talented These Sims are a bit better at all Hobbies. Unlock only using Chef Gino's Talent Charm
Kind of a Big Deal These Sims are better at the DJ Career. Unlock only using Goth Boy's Groove Charm
Active These Sims are a bit better at Yoga
Party Animal These Sims get bonus Party XP every now and then when at parties. Unlock only using Buddy's Party Charm
Alluring These Sims are a bit better at Romantic Events. Unlock only using Yuki's Allure Charm
Outgoing These Sims are a bit better at Friendly Events. Unlock only using Hopper's Outgoing Charm
Competitive These Sims are a bit better at Rival Events
Genius These Sims are a bit better at the Medical Career. Unlock only using Dr. F's Genius Charm
Artisan These Sims are a bit better at Writing and Cooking. Unlock only using Trevor's Artisan Charm
Good Upbringing These Sims are way better at all Events. Bundle of Joy Reward unlock.
How to Unlock Traits
You can simply unlock the majority of the traits by playing with your Sims. Socializing with other Sims also contribute in unlocking new traits.
Aside from playing, you can take the shortcut route by purchasing heirlooms in the store. The in-app store constantly updates the heirlooms you can purchase using Tickets. Heirlooms can unlock traits you don't have in your list. It's not easy to earn ticket to be honest, but you can always purchase tickets using your hard-earned SimCash.