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A "link in bio" (sometimes called a "link tree") is a simple webpage that hosts a number of links to other websites.
These pages can often be designed to match your brand or aesthetic and provide you with a short URL, which makes it great for sharing elsewhere i.e. business cards.
The reason they exist is because some social media websites only allow you to put 1 link on your profile and certain people wish to put multiple.
You can link to your other social media accounts, music, videos, releases, artwork, courses, shows, podcasts or products; the list is limitless.
You can also track analytics such as how many people visited your "link in bio" and how many people clicked on each of your links.
Easy Options
There are plenty of free and paid solutions that allow you to make a "link in bio" such as Linktree (the most common and you get a URL like linktr.ee/yourname), Bio Link (provided by Buy Me a Coffee and you get a URL like bio.link/yourname) and Later's Link in Bio tool (you get a URL like linkin.bio/yourname).
Custom Options
There are also some custom, self-hosted options, giving you the most flexibility and customisation, such as LinkStack (you will need a PHP server to host it) or you can build one yourself with templates such as LittleLink, W3Schools' one or my one and host them on a static server such as GitHub pages, Cloudflare Pages or Netlify (which all have free options).
Some of the easy options and all of the custom options mentioned offer the ability to use a custom domain or subdomain URL for your "link in bio" such as yourdomain.com, links.yourdomain.com, socials.yourdomain.com or more.yourdomain.com. This looks a lot more professional, is better for SEO and prevents being locked-in to a specific provider, in case you want to switch at a later date.