In a PHP-FPM setup, a master process manages a pool of workers. Each available worker accepts a request, runs the PHP code and returns a result. Pool settings control how many workers may exist and how they are created or retired.
Workers matter during patching because they are running instances of PHP. Installing a corrected package on disk does not prove that every process handling traffic has been replaced. Existing processes may continue to use code and libraries loaded before the update until the service replaces them.
The phrase is also used more broadly for PHP background-job consumers. Those workers may run under CLI PHP and need their own restart, separate from the web-serving pool. A site can therefore have several kinds of PHP worker to inventory.
After updating, identify the relevant worker pools and job processes, replace them using the appropriate service or deployment procedure, and confirm that new requests use the intended build.