Each column represents a permission. Hover the mouse over the permission names to get more information about what they represent.
Each row represents a user or a group (often called 'role', depending on the security realm.) This includes a special user 'anonymous', which represents unauthenticated users. Use the text box below the table to add new users/groups/roles to the table, and click the [x] icon to remove it from the table.
Permissions are additive. That is, if an user X is in group A, B, and C, then the permissions that this user actually has are the union of all permissions given to X, A, B, C, and anonymous.