I have an Untitled.fcbbundle file . (associated with Final Cut Pro)

In addition to what Tom said: it seems that you may have used Final Cut Pro and just put everything there in the same library.

Also, due to unfamiliarity with how to manage media, you probably are wasting a lot of space - meaning that most of the size of that "Untitled" library can be easily recovered.

If you don't care about whatever videos you made before, you can trash the whole thing, but if you want to conserve some of that work, open it in FCP and start by deleting all generated files: select "Untitled" inside FCP, do File->Delete Generated Library Files... and check everything. Then quit FCP and check the size of that Untitled.fcpbundle. I bet it is now less than 1 TB, probably a lot less.

If you have Final Cut Pro you could open the library to see if it’s of any value to you.

The Untitled library is created by the application when it’s first opened or whenever it’s reset to its default state.

A library of that size must have a great many media files inside it.