A "link in bio" is a clickable URL placed in a social media profile's biography section to direct followers to an external website, landing page, or other content. This feature is crucial for platforms like Instagram, TikTok, YouTube, etc which do not allow clickable links in post captions, and is used to drive traffic to an online store, blog posts, other social media, or a collection of multiple links. Modern services allow for a single link to lead to a dynamic page with many links, making it a central hub for all of a user's online activities.
This video explains what a link in bio is and how it works on Instagram: