A host string is the name you assign for TNS connectivity in tnsnames.ora. These entries typically identify the machine and port number and [SID]] of the database you are trying to connect to.
If the client and database are on same machine, you don't need a host string.
Here is an example host string as defined in the system's tnsnames.ora file. The entry is named mutation, however you can name it anything you want:
bash-2.03$ tail -7 tnsnames.ora mutation = (DESCRIPTION = (ADDRESS_LIST = (ADDRESS = (PROTOCOL = TCP)(HOST = myhost )(PORT = 1521))) (CONNECT_DATA = (SERVER = DEDICATED) (SERVICE_NAME = mutation))))