All Births, Marriages and Deaths which have taken place in England and Wales, on or since the 1st July 1837 should be have been registered by the state.
At the start, the country was divided into Registration Districts, in each of which where sub-districts.
Unfortunately not all events were recorded. Until 1874 it was the Registrars responsibility to find and record the facts, and it has been estimated that as many as 15% of births went unrecorded.
It should also be born in mind that the registration is recorded in the District in which the event has taken place. So for example Ackerman visitors, who died, will have the death recorded in the District in which they were visiting, and not the District in which they lived.
Full details about what is on a certifcate and how to obtain a copy can be found on the FreeBMD website.
The facts are recorded by year and by quarter. So 1840M refers to the March quarter of 1840.