TSS, NP, IF, and FTP are terms created by Dr. Andy Coggan and are specific to cycling.
FTP - Functional Threshold Power
Functional Threshold Power, or FTP, is the maximum average power you can hold for one hour. So if you can hold 300 watts for one hour, that means your FTP is 300.
FTP is the best single marker for fitness in cycling. The higher your FTP is, the stronger you are.
In TrainerRoad, we use your FTP to scale workouts. Our workouts are recorded as a percent of FTP. This means that when your FTP increases, the workouts get harder to maintain the same difficulty.
Doing an all out one hour effort is really, really hard. It's hard to get an accurate test without it being in a race (much less indoors in a trainer). We use two tests to help estimate FTP; the 20 Minute Test and the 8 Minute Test. These are standard test in the cycling world. We take averages of the all out work intervals in each test and multiply them by a factor, usually .95 or .90. This gives us a good estimate of FTP.
NP - Normalized Power
Normalized Power gives you an average power number if you would have ridden at a constant power output. For example, in a crit, you would have lots of power spikes and lots of coasting. This would result in a low average power. Normalized power was developed to try to figure out how much stress was actually put on your body. By using some math and weighting the really hard stuff more than the easy stuff we can figure out what your power output would have been if you would have ridden steady the entire time.
For TrainerRoad, this number is really helpful because most of our workouts have intervals with rest periods in between. Normalized Power gives you a way to compare efforts between workouts. For example, if one workout has a NP of 223 and another workout has an NP of 243 you put out more power for the second workout.
IF - Intensity Factor
This is how intense a ride is. A ride with an Intensity Factor of 1.0 would equal an all out effort for an hour. If you did an hour at .8 IF that means it was about an 80% effort.
TrainerRoad has the Intensity Factor of each workout specified. This helps you gauge the difficulty of each workout.
TSS - Training Stress Score
TSS is the amount of training stress generated from a workout. The higher this number, the more potential fitness you may have earned from a given ride.
To gain the most fitness, you want to earn the most TSS possible. All of the workouts on TrainerRoad have a TSS score on them. This means that if you were to ride the workout exactly like it is prescribed you would earn that amount of TSS.
How do TSS and IF relate to each other?
Some rides can be intense, some can add a lot of fitness, some do both. A workout that is just 30 minutes long can be really intense and have an IF score of over 1.0, but that doesn't mean you'll earn a lot of TSS from it. By contrast, you can have a 2 hour ride that's not very intense, but will earn you a lot of TSS.