Manual:$wgRunJobsAsync/en

When the execution of jobs during normal page requests is enabled (by setting  to a number greater than 0; it defaults to 1), then this variable controls whether to execute them asynchronously or not.

Details
When running jobs asynchronously, an internal HTTP connection for handling the execution of jobs will be opened, and MediaWiki will return the contents of the page immediately to the client without waiting for the job to complete. Otherwise, the job will be executed in the same process as the returning of the page and the client will have to wait until the job is completed. When the job does not run asynchronously, if a fatal error occurs during job execution, it will propagate to the client, aborting the loading of the page.

Even if  is set to true, if PHP can't open a socket to make the internal HTTP request, it will fall back to the synchronous job execution.

Pages do not appear in their categories
Many users have problems with pages, which do not appear in their categories although they in fact are categorized correctly. In many cases this is a problem with the job queue. The issue people usually have is that pages do not (in fact: not immediately) appear inside their categories. Reasons may be different: It is possible that the job queue is not working at all or that for the way the wiki is being used the queue is not executing enough jobs per page view so that constantly more jobs get queued than are executed effectively stalling execution.

In these cases, one should try running jobs directly with. If they're run successfully and there are no remaining jobs, then one should monitor the contents of the table for a day for example, and see if jobs get stuck there. If this is the case, setting $wgRunJobsAsync to false often helps. Afterwards, the contents of the table can be monitored to check, if jobs are getting executed now.