Each track must have an instrument assigned to it, in order to be heard. By default the first 16 tracks are assigned to the 16 instruments of the first available MIDI device, and the next 16 tracks are assigned to audio.
To change the instrument assignment for a track, right-click on the track label: a pop-up menu will appear from which you can select among all the available instruments.
An instrument can be understood as a single channel on a MIDI device, or as a mono or stereo audio output or an audio synth plugin. Tracks containing note data should be assigned to MIDI or synth plugin instruments, and those containing audio to audio instruments. You can assign more than one track to the same instrument, in which case the multiple tracks will produce sounds in the same way (MIDI tracks playing panned to the left with a flute, or audio tracks playing with the same volume levels, through the same stack of plugins.)
You can change various properties of the instrument associated with a track using the Instrument Parameter Box. This is where, for example, you would set a MIDI instrument to use a particular patch and so produce a particular sound (piano, strings etc).
See the Studio section for more about instruments.