"Digital" is simply a way of coding TV pictures onto a radio wave so that your TV can receive the radio wave and turn it back into a TV picture. The old method was called "analog."
Digital is more efficient than analog and allows a lot more information to be carried on the radio wave. Since the wave can now carry more information, some TV stations are choosing to use that extra information for a high definition (HD) picture rather than a standard definition (SD) picture. In fact, there is so much extra room with a digital signal that most TV stations can now show several different programs at once! Most channels will now have several sub-channels. For example, instead of having just Channel 7, you might have Channel 7.1, 7.2, 7.3, and so on -- each with a different program.
An HD signal takes up a lot of room, so TV stations that broadcast an HD signal have less room to add additional sub-channels.
What is happening with the digital transition is that all analog TV broadcasts will be turned off. Only digital broadcasts will remain on the air. Each TV station decides what it will do with its digital signal. It can use the digital signal to broadcast one HD sub-channel and a few SD sub-channels or it can broadcast a lot of digital SD sub-channels. Most TV stations will choose to broadcast at least one HD sub-channel.