In addition to your own mailbox, INGOS Mail also shows folders that other people have shared with your mailbox. This article explains the mailbox selector in the header bar, the Mit mir geteilt (Shared with me) node in the navigation, and who may see and do what.
Requirement: your own mailbox is linked, see Getting started. Shares are set up by the person who owns the mailbox; shares cannot be created in INGOS Mail itself.