That's the census reference number, which can be used to identify any page in any census.
In this case:
RG 9 identifies the 1871 census
3134 identifies the 'piece number'
28 identifies the folio number
49 identifies the page number.
It sounds complicated, but in fact once you've got used to it it's fairly easy to cope with. Giving it makes it much easier to find pages really quickly.