Link in bio refers to adding an external URL leading to a website, product description, or any sign-up form in your social media profile. It is a call to action to the customer to either visit your website or check your site page. Most social media platforms do not allow links to be attached to any post; hence, a link in the bio is the only clickable URL that can take the visitor to any important site. 

Your bio can integrate any link you want to, but to engage the audience productively, linking to any latest blog or product will do better. 

Example 

The YouTube link is mentioned in the bio of Social Champ’s Instagram page.