Contentlayer is the glue between your content and your code.
For many years it's been commonplace to separate your website code from its content. This makes both content editing and developing more efficient and prone to fewer bugs. (And who can argue against code without bugs?)
The code for your website is often built on top of a front-end framework or static site generator, such as Gatsby or Next.js (among many, many others). And the content is often either housed in a headless content management system like Contentful or Sanity (also among many others), but can also be stored in local files.
Some frameworks are opinionated about how your content should be loaded into your code. For instance, Gatsby has a large marketplace of plugins and a GraphQL layer that can be used together to load content into your code.
Other frameworks like Next.js take no stance on content, and leave it up to you. That is where Contentlayer shines.