Strapi plugins allow you to add extra functionalities and custom features to power up your Strapi application. To build a custom Strapi plugin that publishes contents to Medium and Dev.to using their APIs from the Strapi admin panel, we will create a new Strapi plugin using the command `npx @strapi/sdk-plugin init content-publisher`. We will then define the schema for our Post collection type and customize our Strapi plugin server by creating routes, controllers, and services. Our plugin will allow us to publish blog posts to Medium and Dev.to while also displaying a table of published posts on our admin panel.