May I know What is this key symbol ?

These can be created if you import a folder with clips in it. The folder name is used to assign the keywords to the clips. It’s a feature.

It indicates a keyword collection.

Keywords can be assigned to clops or ranges within clips and the respective keyword collections are automatically created. It is a powerful media management feature of FCP.

To better familiarize yourself with this and morr, I suggest the following great educational videos:

Learn FCP in under 60 minutes by rippletrainhttps://www.izzyvideo.com/final-cut-pro-tutorial/

Izzyvideo introductory course:

https://www.izzyvideo.com/final-cut-pro-tutorial/

These are created when a keyword is assigned to a clip.

You can delete the keyword collection by selecting it, control-click, Delete Keyword Collection.

This does NOT delete the clips, it just eliminates the keyword and the respective collection.

Please show me the import screen.

I think Tom is right, and the ability to use folder names as keywords is actually useful, but if it is not useful to your workflow it can be turned off in the import settings.

You can turn off the "create keywords from folders" function in both the Settings (only affects drag-and-drop clips) and the Import window (affects only those imported via that window).