Because HDTV signals are digital, extra information can be placed into empty spots in the data being transmitted. This allows the TV station to broadcast more than 1 channels worth of information. For example, WKMG broadcasts its Doppler radar map real time on a sub channel. WFTV broadcasts news headlines, Doppler radar, traffic reports, weather forecasts and has a small window which contains the programming on their main channel.
TV stations are free so use the extra space in the HDTV signal
as they wish. Only time will tell how creative TV stations will
become. There can be trade-offs between the main channel and the
subchannel; the quality of the main channel can suffer if there
is too much data on the subchannel.
The TV station's main channel has a -1 attached to the channel number, for example WFTV 9-1 is the main broadcast channel. WFTV uses 9-2 for their
radar channel. (9-0 is currently reserved as the channel number for NTSC broadcast so HDTV receivers can differentiate the two broadcasts because they share the same main channel number. WKMG uses 6-1 as their main broadcast channel and 6-2 as their radar map channel.
WESH use 2-1 as their main channel and shows NBC Weather Plus on
2-1. A station is fairly free to choose sub channel designations as they see fit.
The FCC does not require cable companies to carry subchannels. Cable is negotiating to add the subchannels being broadcast by the local stations on a case by case basis.