Realm permissions control which areas of Cacti a user can access. You can edit a user's realm permissions by selecting User Management and choosing the user you want to edit the permissions for. The Realm Permissions box will be displayed in the lower part of the screen. Each "realm" is a grouping that represents common tasks in Cacti, making it easier to fine tune each user's access.
If you want to create a user that can only view graphs, you should select the View Graphs realm and leave everything else unchecked. See the Graph Permissions section for more information about how to fine tune this even more on a per-graph basis. Conversely, if the user needs to access the console, they will need Console Access and any additional realms that you see fit.