As usual, half of the answer is in the question...
1/. The convention of Chicago was held in December 1944 in... Chicago
! It was the first major step towards standard rules & procedures in civil air transportation (+ navigation). That's where the ICAO comes from.
2/. To ratify basically means "to sign because you agree with what's on the paper". In this case, it means that all the States who agreed to follow the ICAO guidelines signed the convention. If for some reason a particular State does not want to follow all the rules, it has to notify ICAO of its "Differences", so that everybody' aware of them.