That means you’re trying to read from a secondary node in a replica set, you can only read from the primary node by default.
You can allow a secondary node to accept reads by running rs.slaveOk() in a mongo shell that is connected to that secondary node. Allowing reads from a secondary is not recommended, because you could be reading stale data if the node isn’t yet synced with the primary node.
UPDATE: As Janusz Slota’s comment points out, rs.slaveOk() is no longer used. Use rs.secondaryOk() instead, however this is still not recommended. Here’s the documentation for rs.secondaryOk().