It is the predetermined action that the website owner or the marketer wants a website visitor to take on the website.
For example: Suppose you have an e-commerce website where you are selling shoes. So your predetermined action for the visitor will be to buy your products.
Or
If you have a blog website, you want your visitors to read, share, like, comment and subscribe to your newsletters.
If the predetermined action completes then, it’ll be called Conversion.