While training for my Ironman and half-Ironman races, my coach steered me away from FTP tests. He felt they just tested how good you were becoming at doing an FTP test.
Coach Joe Beer felt the ramp test I regularly performed gave him a better metric from which to advise the power output at which I should ride events of different durations. He was always spot-on.
It's a measure in average Watts of how hard you can ride for an hour. In practice, however, most FTP tests last twenty minutes then a formula is used to scale your average power down to an hour.
It's been rather wet recently, so I've found myself back on Zwift, the virtual cycling world. It has two FTP tests built in, and comes with a set of training programmes. So I clicked on the FTP Tests (shorter) version and had a go. 189 if you're interested.
Not great, but considering I haven't really trained since my knee injury last September, I'll take it.