What does 232 signify in the RS232 standard?

It signifies nothing; it's part of a sequential list of EIA standards: -

It's the document serial number of the standard. Same reason why the HTTP protocol is also known as RFC2616 and the Javascript programming language is also known as ECMA262.

The numbers themselves have no meaning. For example while EIA232 specifies the electrical characteristics of a digital serial communications system, EIA222 specifies standards for antenna masts and RS225 is a standard for RF connectors.

Wikipedia has an incomplete list of popular RS/EIA standards: https://en.wikipedia.org/wiki/EIA_standards

RS = recommended standard

RFC = request for comment,

numbers don't have any real meaning