Plain English Meaning
One radio and one needle that can work with two different kinds of ground stations: VOR stations used for general navigation, and localizer transmitters used for lining up with a runway on an instrument approach. You pick which one by tuning its frequency.
Full Definition
A combined navigation receiver and indicator capable of displaying guidance from either a VOR (VHF Omnidirectional Range) station or a localizer transmitter, with the pilot selecting the appropriate frequency for the navigation aid in use. On an HSI, VOR/Localizer information is presented through the same course deviation indicator, with the instrument automatically interpreting the signal based on the tuned frequency.