If there is another anime which should be watched before or after watching a certain one, you should add it using this sequel/prequel feature. This helps people who want to watch the anime stages in the correct order.
On the top of the page you see the sequels/prequels for this anime. Specify the anime you want to add as sequel/prequel by anime title, synonym title or short title. Select the watching order (i.e. is the anime you entered a sequel (has to be watched after) or a prequel (has to be watched before)). Now you have to select whether you want AniDB to automatically add the current anime to the anime you just entered as sequel/prequel too. Or if for some reason you want to do it manually. If unsure always let this checked, which is the default.
If the anime entry is not yours (you haven't created it) you can't remove relations. Please use the DB Change Request forum in that case.