At its core, the FeministWiki doesn't save anything about you other than your username, your password, and who added you as a member.
Your username needn't correspond to your real name. If it does, note that other members can see it in some places such as the list of names in the chat service: https://chat.feministwiki.org/ Non-members cannot see a list of all members, but if you edit the wiki, post on the forum, etc., then your FeministWiki username will be publicly visible. If this is a problem for you, please contact the technician to change your username to one that doesn't relate to your real identity.
Your password is not saved in plain text. Rather, a "salted SHA1 hash" of your password is saved. This means that even in case of a data leak, attackers won't immediately know your password, though it's technically possible for them to figure it out if they try very hard. For this reason, the password you use here should not be a very important one, such as the password you use for online banking. (This issue applies to almost all websites that use passwords; the FeministWiki is not any less secure in this regard than other websites.) All FeministWiki services use encrypted communication, so your password doesn't travel over the network in plain text either.
Who added you as a member is only visible to the technician, if she/he cares to look it up from the internally kept database.
In your user preferences page, you can set an e-mail address that can be used for password recovery. This e-mail address is visible to the technician.