FTP = how hard you can go for an hour, measured in watts.
Watts rate of doing work. You can work very hard for short periods (sprint) or amble along gently with low effort for much longer.
A novice cyclist will be able to sustain about 200 watts for an hour. Of course this depends on how big you are. 200 watts is a lot for a 45 kilo lady, and not much for a 150 kilo man. So watts/kilos is a fairer measure for a lot of riding.
There is a lot of theory and argument about how to train – improve FTP, improve endurance, etc. But generally; more riding makes you fitter and able to work harder for longer. Training at or even above your FTP (for short periods, not an hour) will make you fitter faster.
Beginners will have about 2.5 Watts/kg
Fitter cyclists 3 Watts/kg
Trained cyclists 3.5-4 Watts/kg
Successful racers 4.5-5.5 Watts/kg
Pros 5.5-6.5 Watts/kg
6.2 Watts/kg wins you the Tour de France. Pros are about 30% more powerful than amateurs.
Last point, there are no shortcuts, but the biggest gains come early. The jump from 4.3-4.5 is not coming at all.