'WIITWD' is an acronym for 'What it is that we do'.
It was coined to be a very general term and to help avoid the arguments among people who prefer different terms for the most general 'catch-all' term. Historically, some people prefer to use 'SM', some prefer to use 'BDSM', and some prefer to use 'leather'.
A Personal Note from Robin
For the pioneers/old scene folk, it is an acronym for 'What it is that we did'.