Think of it like this:
Vatican II was an attempt to change the way the Church teaches the faith to make it more understandable by the modern man. The Council was hijacked by liberals and many of the documents produced by the council contained ambiguities - which were exploited by mis-interpretation by liberals to push forth their agenda of destruction.
Rome calls for the documents to be interpreted in the light of tradition, not as a break with the past. However, many claim, including the SSPX, that some of the documents are outright wrong - the document ‘DIGNITATIS HUMANAE’ seems irreconcilable with previous Church teaching, for example. For this reason, the SSPX rejects the documents of Vatican II. They accept that it was a valid Ecumenical Council, of course, but they reject the documents promulgated by it.