understanding the use of MTBF
The essential thing to understand is that it's statistics - which you graze with the statement:


  does not mean 1 unit will run for 1 million hours. If however I had 1 Million units I think it means I should expect a failure once an hour?


Statistics tell you NOTHING about what an individual item (or event) will do, but a great deal about what a population of them will do - subject, of course, to the "garbage in, garbage out" rule - if the data they are based on is accurate, they are accurate for a population - if the data is flawed, the statistics based on it are flawed. 

In practical terms there are other factors of how real life behaves, such as the "bathtub curve" - there's a disproportionate number of failures early (from e.g. manufacturing defects), fewer over the major lifespan of the product, and a disproportionate number of failures late in life from aging (all of which are reflected in the MTBF, but the bathtub curve is normally seen when looking at the actual failures that make up the mean.) If you've heard of products being "burned in" that's effectively trying to catch the first part of the bathtub curve before the product hits the marketplace, which makes the perceived failures in the wild look better (and you get less bad press of the "I bought a brand new XYZ widget and it broke the first day I used it" sort.)

If most of your products are "throw away if they break" then MTBF is MTTF. Or MTFF is MTTF (where MTFF would be "first failure")