For the OP. FTP = Functional Threshold Power LTHR = Lactate Threshold Heart Rate These terms are somewhat interchangeable but as the metric is different there will be day to day differences. That's what Vesteroid is talking about in post #44. IMO These terms become important after one has trained enough to risk over-training. For easy math purposes lets say one's Max heart rate is 200 which would be typical of some in their early 20's Riding hard for an hour after warm-up they average 150 beats/minute. Their lactate threshold is 75% of Max. Let's say they follow one of the expert's training plans. A year later they can average 160 so their lactate threshold has improved to 80% of max. The concept of lactate threshold allows the training plan to adapt to the progress of the rider.

Most of the people that begin following one of these plans say the difference is that the plans call for spending more time riding slower so that the fewer times spent riding hard you ride a lot harder.