Manual:$wgTransactionalTimeLimit/en

Details
Typically, web requests should be rather quick, and PHP tends to have a 30 second default timeout, which is fairly high. MediaWiki, however, can sometimes by very slow with certain operations, such as page saves, and these operations may furthermore involve updates to multiple datastores that ideally should all happen or not happen at all. All POST requests set  to avoid client-initiated aborts causing half-written data. Certain requests call a special method that also raises the time limit to  to both give the requests a chance to finish (as it might be slow) and to avoid timing out when only some of the writes where done.

By having this value, site admins can use fairly low timeouts to help avoid DOS and wasted threads, but avoid half-written writes due to timeouts at the same time.