Functional Threshold Power (FTP) is defined as the greatest mean maximum power you can currently produce for one hour on your bike. And FTP is without doubt one of the most important body metric along with heart rate, w/kg and cadence.
This is important because it provides you with precise training zones, so you always get the maximum value from your time in the saddle.
Most of us weekend warriors tend to attack our cycling with a mix of under- and over-training. Under-training means you don’t push yourself enough, and your fitness won’t improve much over time. Over-training means you push yourself too hard, prolonging your recovery and fitness gains along with it. FTP provides you with precise training zones, so you always get the maximum return from your time in the saddle. And since FTP is a moving target it also serves as a good marker of how your training is going. You know when your FTP is rising that your fitness is improving and your training is going well.