This program uses the MIDI protocol, so it is a good idea to to have some basic notions about MIDI in order to fully understand the concepts behind it. You can find here a good introduction: What is MIDI.
KMetronome produces MIDI events. If you want to hear the events translated into sounds you need to connect the MIDI OUT port from this program to the MIDI IN port of a MIDI synthesizer. It can be either a hardware MIDI synthesizer or a software one. If it is an external hardware synthesizer, you also need an ALSA supported MIDI interface installed in your computer, and a MIDI cable attached to both the computer's MIDI interface, and the synthesizer MIDI IN socket.
If you don't have an external MIDI synthesizer, you can still use the program with a MIDI software synthesizer supporting ALSA sequencer connections, like QSynth. Start your software synthesizer before KMetronome, and then make the connection between the two programs, either on KMetronome's configuration dialog or using an external connection manager like QJackCtl.
Don't forget to install a good Sound Font into QSynth's "Setup..." dialog.