Galaxy is a web application that uses handlers to perform actions.
There are two main types of actions that are carried out by handlers:
Respond to user requests; These actions are carried out by web handlers
Manage the execution of tools; These actions are performed by job handlers.
By default, Galaxy is configured to run a single handler that handles both user queries and jobs.
Depending on the number of users accessing your Galaxy instance or the number of jobs you need to manage you may need to start web handlers or additional job handlers.