good evening
novice on ubuntu, often when I pass the cursor over the activity manager or thereabouts,
session crashes
What's going on?

Which version of KDE are you using? This can be found in Help > About KDE in any KDE application (such as KWrite or Konsole).

Also, are you able to use the KDE Crash Assistant (Dr Konqi) to generate a backtrace and post that here? You can find it on the Developer Information tab of the assistant, which should open when the crash occurs.