The lock icon next to the names of certain files in the Sdui cloud belongs to the "collaborative working" feature.

This symbol can only be found for file formats that could in principle be used for joint editing of the file.

The closed lock means that the file is not approved for editing.
Each file is initially marked with the closed symbol and must be released for editing manually by the group admin.

The open lock means that the file is shared for editing. In this case you can click on the three dots to the right of the file name and click on "Edit content" to be forwarded to the online document.