A Tableus Fetcher is a package that enables tableus to communicate with your backend. It should be able to receive data from the server, given a certain table state, and transform it into the form that tableus expects it. The table state consists of the current state of the pagination, filters and sorts.
A valid Tableus Fetcher has to provide a fetcher class, that implements the fetcher interface (source). The fetch method of the fetcher class is given the table's current state and configuration and should return an array of data objects, as well as, updated information on the pagination state from the server.
For an example you can check out the laravel-rest-fetcher UI.