Ok, there appears to be something to this.
The description of how they claim it works is pure word salad. Nevertheless I've been surprised checking this out, this would appear to be one of those rare cases where they've picked up on something that's actually getting used in cancer treatment. It does not cure cancer but it may be part of a useful treatment.
So, on to the science.
"Frankincense oil derived from Boswellia carteri induces tumor cell specific cytotoxicity"
"Frankincense essential oil prepared from hydrodistillation of Boswellia sacra gum resins induces human pancreatic cancer cell death in cultures and in a xenograft murine model"
Now these 2 are about killing cancer cells in the lab and we all know the problem with that:
Now I'm going to focus on "monoterpene perillyl alcohol", one of the monoterpenes in question.
With a quick search I was able to find this trial:
"Efficacy of monoterpene perillyl alcohol upon survival rate of patients with recurrent glioblastoma[brain tumor]"
Patients with recurrent primary GBM treated with POH survived significantly longer (log rank test, P < 0.0001) than untreated group. Patients with recurrent primary GBM in deep location survived significantly longer than with lobar location (log rank test, P < 0.0001). Median survival rate of secondary GBM was 11.2 months, longer (log rank test, P = 0.0366) than primary GBM (5.9 months). Radiographic improvement and reduction of corticosteroid dosage (36%) further associated with a delay towards progression.
So specific monoterpenes may actually be useful in cancer treatment.
And also this, against:
"Failure to demonstrate chemoprevention by the monoterpene perillyl alcohol during early rat hepatocarcinogenesis: a cautionary note."
In conclusion, PA exerted no detectable chemopreventive effect in the early stages of rat hepatocarcinogenesis. It rather exerted a PB-like tumor promoting activity. These data argue against a recommendation of PA as a chemopreventive agent for healthy humans.
So you probably don't want to take it routinely while healthy.