Can someone explain exactly what affordable housing is?

Has anyone on Brighton and Hove City Council looked back to the rules on housing in the Fifties and Sixties?

It was known that once the children of a family had left home the parents were moved to smaller accommodation thus freeing up three and four-bedroom houses for people on the housing list.

I wonder how many homes are occupied by one or two people that could be put to better use?

-Mrs P Twiselton, Hove