@CottonTales is referring to the fact that all MSTP MD/PhD programs are fully funded by federal grants --that is, students in MSTP programs have 100% of their tuition (med school + grad school) paid for and they also receive a modest living expenses stipend (typically ~$30K/year) during both the med school and grad portion of the program.
Although there are only a handful of non-MSTP MD/PhD programs, even those still pay for tuition and living expenses for MD/PhD students. Non-MSTP program are funded by grants from academic and industry sources. (drug & biotech companies mostly.)