The TikTok algorithm is a recommendation system that determines which videos will appear on your (unique and highly personalized) For You page.
No two users will see the same videos on their For You page, and the videos you see might change over time based on your viewing preferences and even your current state of mind.
Here’s how TikTok itself defines the TikTok For You page algorithm:
“A stream of videos curated to your interests, making it easy to find content and creators you love … powered by a recommendation system that delivers content to each user that is likely to be of interest to that particular user.”