Focal therapy is another term for ablation. Focal draws attention to the fact that it is targeted. Ablation draws attention to the fact that it kills the cells in situ (on site) rather than removing them (prostatectomy) or suppressing them (ADT).
With respect to prostate cancer, this generally means identifying cancer that is localized (prostate cancer generally forms nodules) and well-contained (there is one or a few spots of concern, not many) in the prostate.
Metastasis, in contrast, means the cancer has spread beyond the prostate itself. The type of cancer cells can be identified as coming from the prostate, but they are growing other places--eventually in the bones.
Not just @bjroc, but many researchers have longed to add focal therapy to their arsenal of cancer treating strategies. So far, it has not become a front line treatment. Why?
One, as various contributors have mentioned it's limited. While many techniques have been tried to kill prostate cells in a specific location, none have been entirely successful. (Radiation, radioactive seeds, other implanted "seeds", lasers, freezing, ultrasound, specialized radiation like proton beams, and even more.) Obviously part of the problem is knowing what to destroy and part of the problem is precisely targeting tiny spots in the very middle of your abdominal core. Breasts, in contrast, are much more accessible.
Two, the equipment is incredibly expensive. This means that each new technique or variant requires huge investments of capital, and once this has been done, huge marketing efforts (by the few who have the equipment) are required to reap a return that might pay for these investments as well as the specialized practitioners. Posting information online and sponsoring non-profit foundations to get the word out can be part of those strategies. (As an analogy, right now an investment company sponsors adds discussing how to choose between their three much larger competitors! They want you to trust them to help you make your decisions. This could conceivably happen in medicine, even in cancer medicine.)
Three, ablation (focal therapy) creates scarring. This scarring--the dead cells left behind--confounds the future monitoring and treatment of the cancer, which is likely to recur, since even if the cancer nodules are eliminated, the same prostate that formed them is still there.
Four, as with all prostate cancer treatments, the often slow development of prostate cancer frustrates evaluation of treatment alternatives. When active surveillance is a reasonable option, you know that outcomes must be in doubt. Very high numbers of men with prostate cancer live 5, 10, and 15 years. Most get it after age 50--the average age is mid-60s. This makes evaluating treatment options a lot tougher than if people die in 18 months. My sister's brain cancer "breakthrough" turned 18 months into 36 months (average from diagnosis to death.) That makes research easier than turning 10 years into 20 years, especially since people also die from other causes.
So why do ablation/focal therapy? Because while PC has no symptoms for a long time, PC treatments affect poop, piss, and passion. The literature refers to this as health-related quality of life (hrQOL etc). It's a step more aggressive than active surveillance, which can extend for years, but less aggressive than RP, ADT, and more extensive ablation (less focal if you will!)