Host (baseband) controller in mobile phone
Typically the 'host' refers to the master device/entity that belongs to the 
side of the computer. It could be inside computer or external to a computer.
[it could be a microcontroller , but the point is host is more closer to you
when you writing software to work with].

The main idea is before you communicate with the other device, you communicate 
with the host, for a example hard disk controller is a host, USB controller chip
is a host.

So according to your GSM module thing, The chip hardware/software that parsing the
AT command set and act as a front end is refereed as a host.

Your Manual/Specification probably should have a chapter called terms and Abbreviations.
Turn out for that page and you'll find the specific abbreviation which is specific to
your module.