Access rights define who is allowed to do what with the content that you put into SyneMind.
By default, whatever you create (notes, tasks, uploaded documents and media) is yours only, and you do not need to worry about access rights. None of it can be accessed by anyone else.
However, if you are a member of a group, you can "share" content into the group. Others will then gain access to it, depending on their status within the group: ordinary members will only get "view" permissions (see, but not edit), while content editors will be allowed to change the content (which also includes the permission to delete it). You will retain full access to the content (read / edit / delete) even if you are only an ordinary member of the group and not an editor.
When you are viewing an article (note or task), you will see a permissions and sharing panel in the top right telling you what you can do with it, and if it has been shared into groups. If you are content editor of a group (or the article "belongs" to you to begin with), you will be able to un-share it from that panel. In order to share, you will need to be content editor.