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 ( or service name) 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.
The connect string, a synonym for host string, includes the opening and closing brackets, i.e. all the letters righthand of the equal-sign.
If you use it on commandlines from any shell in a Unix/Linux environment, it maybe necessary to embed the host string into apostrophes.
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))))