Those are memories that a sim recieves for achieving their "First" of something. They're just for fun. Here's a list I found! (:
Birth of a child
Adopting a child
Learning how to walk
Learning how to talk
Being potty trained
Teaching a toddler to walk
Teaching a toddler to talk
Potty training a toddler
Got an A+/D Report Card
Child got an A+/D Report Card
Growing up well/badly
Meeting a Sim
Becoming friends with a Sim
Becoming BFF's with a Sim (Freetime)
Becoming best friends with a Sim
Losing a Sim as a Best friend
Having first kiss with Sim
Being rejected for first kiss
Kissing a Sim for the first time
Making out with a Sim
Being rejected to make out with Sim
Relative got engaged
Relative got married
Getting engaged (can be good or bad)
Got Rejected for Engagement Proposal
Getting married (can be good or bad)
Got Left at the Altar
Relative Got Left At the Altar
Sim joins the family
Getting a child taken away by a Social Worker
Getting a grandchild
Learning to study
Teaching another Sim to study
Went to College (University)
Made Dean's List (University)
Other Sim made Dean's List (University)
Went to Academic Probation (University)
Completed Freshmen/Sophomore/Junior Year (University)
Graduated with or without Honors (University)
Never Went to College (University)
Failed College (University)
Getting abducted by aliens
Falling in love with a Sim
Having ex-lovers
Going steady with a Sim
Being rejected to go steady with a Sim
Having an affair with a Sim
Getting caught cheating by a Sim
Breaking up with a Sim
_______ Broke Up
Obtaining a pet (Pets)
Meeting a pet (Pets)
Being chased by a pet (Pets)
Becoming a PlantSim (Seasons)
Becoming a Vampire (Nightlife)
Becoming a Werewolf (Pets)
Seeing first snow (Seasons)
First date with a Sim (Nightlife)
Having a bad date with a Sim (Nightlife)
Having a great date with a Sim (Nightlife)
Having a dream date with a Sim (Nightlife)
Sim dying
Getting a promotion
Reaching the top of a specific career
Maximizing a skill
Maximizing all skills
Getting a job
Getting fired from a job
Quitting a job
Learning how to make a certain food
Fire!
Vermin!
Becoming enemies with a Sim
Having # friends
Saving another Sim from death
Failing to save another Sim from death
Have first WooHoo
Doing WooHoo with a service Sim
Doing WooHoo with a robot
Had WooHoo in public
Ate Grilled Cheese (Sims with Grilled Cheese aspiration only)
Golden Anniversary
Retire
Repoman
Promotion
Demotion
Got Fired
Had a big bonus
Sneak Out
Got caught sneaking out
See Ghost
WooHoo with Sim for the first time (University)
Changed Aspiration (Nightlife)
Had a good reputation (Apartment Life)
Became Big Man/Woman on Campus (University)
Opened First Business (Open For Business)
Got a Good/Bad Review
Owned a Top-Level Business (Open For Business)