These records first became publicly available in 2010 but only via an expensive service that gave you who was living at a nominated address - more details here.
The records for Scotland and for Northern Ireland are held separately and are not included in this release - again more details here.
The 1939 Register is even more critical for family historians because the 1931 census was lost in a fire and no census was taken in 1941 so after the 1921 census becomes available in January 2022, the next one after that will be the 1951 census, on current rules only available in January 2052.
The 1939 Register for England and Wales contains information on 40 million individuals. However because of Data Protection rules and the belief at the time that records would be undisclosed for peoples' lifetime, the decision has been taken that only the records of those over 100 years ago will be visible at Findmypast which means initially only those born in 1915 and earlier, that is aged approximately 24 and older at the outbreak of World War II. Findmypast have promised to regularly release more records as the years pass.
There may also be gaps because it emerged subsequently that in some areas a significant number of mothers had omitted their sons from the registration because they did not want them to be called up for National Service but later came forward to register them when they realised it was needed for ration books. It is likely however that a large proportion of these would not have been available to view initially because they would have been born after 1915.
The register was compiled on the night of Friday 29th September 1939 and the following details were recorded for each person:
There is a lot more detail about the background and collection of the 1939 Identity Card Register here as well as the subsequent history. The Identity Card was finally abolished in February 1952, but the identity numbers were used within the National Health Service to give everyone an individual number and anyone who had a national identity number during the Second World War or just after still had the same number as their NHS number until a few years ago.