KDED stands for KDE Daemon which isn't very descriptive.
KDED runs in the background and performs a number of small tasks.
Some of these tasks are built in, others are started on demand.

### Built in tasks

* Checking for newly installed software and updating ksycoca when new
  software is detected. Updating of ksycoca is done by the program kbuildsycoca
  which gets started by kded. When kded is first started it always runs
  kbuildsycoca to ensure that ksycoca is up to date.

* Checking for newly installed update files. Applications can install
  *.upd update files. These *.upd files are used to update configuration files
  of users, primarily when new versions of applications are installed with
  (slightly) different configuration file formats. Updating of configuration
  files is done by kconf_update. kded starts kconf_update when it detects a
  new update file. When kded is first started it always runs kconf_update to
  ensure that it has not missed any update files. kconf_update keeps track
  of which update files have been processed already in the config-file
  kconf_updaterc. It only performs a certain update once.

Homepage:
https://community.kde.org/Frameworks
