Almost always, this is caused by a low battery. A typical liquid battery will only last for 1-2 years, after that it doesn't charge up like it used to. If your battery is weak either from age, or if you left the lights on or something, commonly it will still have enough juice to activate the starter solenoid (thats the clicking sound) but not enough to move the starter motor and the engine.
This can be confirmed simply by doing a voltage test on the battery with your multimeter. If it is below 12.4v, the battery will struggle to start the bike. If it is below 11.5v, you may only hear that clicking sound.
Don't worry though! You may still be able to start up your bike.