MediaWiki:Interfaceconcurrency-desc/qqq

A "backend" is a system or component that ordinary users don't interact with directly and don't need to know about, and that is responsible for a distinct task or service - for example, a storage back-end is a generic system for storing data which other applications can use. Possible alternatives for back-end are "system" or "service", or (depending on context and language) even leave it untranslated.