Projects shared with you and of which you are not the creator are indicated by an arrow to the left of the project name (in the menu bar on the left).
This means that you haven't created this project and that someone else has shared their project with you. Depending on the rights you have been granted, you can more or less edit the project.
Please note that any modification made in a shared project is visible to all people with access to the project.
To see what your rights are, go to the project settings: Project> "Settings" icon in the top right corner of the window. Go to the "Collaboration on project" section. There, you can consult the list of members having access to the projects, as well as their rights.
To learn more, see the article dedicated to project sharing and access rights.