This is an example of the full Census Reference given for Bryanston Square, Upper George Street, Marylebone in the 1851 Census:-
1-21 HO107/1489 518-525
23-26 HO107/1489 649-651
28-48 HO107/1489 525-531
1489 is the Census Piece and 518-525, 649-651 and 525-531 are the census folios the address can be found on. As Marylebone is such a vast area, the enumerator would have had a lot of individual books that when filled in were collected and later bound into a single book so it was easier to access. The bound book is called the census piece.
The folio number makes it easier to find the address and is at the top right hand corner of every other census page. The sheets in the original books the enumerators used were double sided so the folio number is only listed on every other page.
It was not unheard of for the person stamping the folio numbers on the page to make a mistake and either stamp consecutive sheets with the same number or to miss stamping a sheet altogether so you sometimes see 6a or even 6b for example.
A page number is also listed, but this should be ignored as this is the page number from the enumerator's individual book as stated above.
The writing on Census Returns is not always easy to read, particularly in earlier returns. If you are consulting census returns online, please remember transcriptions may be incorrect. The census was sometimes written in pencil and has become illegible over time.
Spellings of surnames have also changed over time e.g. McJannett, McJanett, Jannet, Jannett etc.
There is a street index for most English and Welsh towns and cities, but there is no index for villages and so the census reference includes the piece number but not any folio numbers.
One of the best ways to find out more about the Street Indexes available is to go to the Historical Streets Project. It is a good source of information, but is now read-only. Only the 1841, 1851, 1861, 1871 and 1891 census returns have been indexed. They are listed by Registration District, so you do need to know the rough area in which the street you are looking for was situated.
The National Archives is in the process of making their books of street indexes for the 1841-1891 census returns available on-line.