Its not needed on a single node. In the latest sandbox, this process has been turned off (maintenance mode) to conserve resources.
On the sandbox page, see here for sandbox release notes for the current sandbox under section "Services not started automatically".
HDFS
SecondaryNameNode
*Since on a single node, secondary namenode is not needed, it is not started.
For more details on what secondary name node does see here
If you are bored and have some time, this video also helps explain in way you won't soon forget 🙂