Most relative clauses refer back to a noun phrase in the preceding discourse. For example :
I spoke to John, who agreed to do it. (Who refers back to John).
I liked the last book that she wrote. (That refers back to the last book)
Various relative pronouns and adverbs can be used in relative clauses like this - who, which, that, whose, whom, where, when etc
A sentential relative clause is one which does not refer back to a single noun phrase, but is a comment on a proposition ("idea") which has already been mentioned in the discourse. Examples :
1. He agreed at once, which surprised me as I didn't think he'd like the idea.
2. A: John's free then - he could do it. / B: Which would solve the problem.
As the examples show, in spoken English, the sentential relative clause may be within one utterance, or in an utterance by a second speaker, but always refers back to a previous proposition (here, in (1) "his agreement" or in (2) "John being able to do it".)
Which is by far the most usual relative pronoun found in this type of clause.