Level 15 is the least secure in that it allows all types of router commands to be used. Level 1 is most restrictive in that it only allows display type and non-critical commands. One way to tell what you can do at your level is to put a ? at the command prompt or after you enter a command to see what you have available.
If you are wanting to give others access to the router, such as a help desk, but do not want them to have the full array of commands use level 1 or level 3 and then if you have specific command(s) you need them to do that level 1 or 3 does not allow you can then override the level of a command by using the privelege statement for that specific command.
Dave