The AT300 (presumably shortened from A-Train 300) is the family of high speed rail vehicles made by Hitachi Europe. Class members include the 395s, 800&801s (procured as part of the Intercity Express Project) and the new hybrid units for FGW West of England routes.
Other classes of Hitachi train include the AT100 and the AT200. In a way, I suppose it is a bit like Siemens having their desiro family, and their velaro family