For a MTBF of 2 million hours, that means that on average, if you have one thousand drives, then you should expect one drive failure every 2000 hours, or 83 days (1k * 2k hours = 2M hours)
Of course this breaks down at an MTBF of over 50 years, as thosre rarely mention exotic failure modes, and don't actually have an MTBF of 50+ years over the life, but an annualized failure rate corresponding to 50+ years MTBF, measured over the first couple of years or even the first year. For non-wet-electrolytic-capacitor-using computing, one can calculate a temperature-dependent MTBF in the 5-25 years range, mostly depending on how bad the chips are hit by electromigration and similar aging in the semiconductors. This is incidentally a reason why I miss clock speeds for different processors as reported by overclockers to at least in some cases extrapolate the life due to electromigration, as there is a formula with like iirc 2 parameters, which gives a temperature (and maybe voltage) dependent lifetime/MTBF for this semiconductor device. I'd likely sttrive for about 5 years MTBF on the processor, if speed is of concern and reliability/uptime not in the foreground.
That assumes your drives fail with a constant independent probability, like nuclear decay events (Poisson distribution). The reality is more like https://en.wikipedia.org/wiki/Bathtub_curve .
MTBF is not a good metric for complex systems used under wildly-varying load conditions, but ... it's a metric.