Right now, the web admin is really kind of view-only. In progress is a web-based playlist editor, and hopefully more web-based admin.
You can’t really disable it — the webserver is necessary for serving the audio, so you can’t turn it off or anything.
Generally, though, it’s just informative… how many songs in the database, who is currently listening to songs, etc.
You can get to it by http’ing to the port you have set up as the daap port, usually 3689:
You’ll be prompted for a username and password. Username can be anything, password is the “admin_pw” in the conf file.