Hi! "it" is refering to the "individual visits". It is saying "the individual visits are the most important things" by stressing on the "indvidual visits". Probably there were other things listed before that are not so relevant for the speaker

Linguists call this use of "it" the "dummy it". The basic sentence is "Individual visits are the most important." But English speakers use the "dummy it" construction to put extra focus and emphasis onto part of the sentence - making it a "topic". For example: John ate the cake. (canonical structure; not strong emphasis on anything) It was John who ate the cake. (emphasises the person who ate the cake) It was the cake the John ate. (emphasises the object that was eaten) These are used in different contexts. E.g: A: Oh my god! Mary ate my birthday cake! B: No she didn't! It was John who ate the cake. A: Oh my god! Did john eat my sandwich for his dinner? B: No, no. It was the cake that John ate. In your sentence, the author is basically saying that "individual visits are the most important" but uses the 'dummy it' construction to add extra emphasis on 'individual visits' compared to other possible factors that the reader might think are important.

联系上下文，通篇理解。这里的it不是指代，而是一种说法，特指it is 后面的事物，算是强调吧。Individual visits are the most important. 这样读能读的出来强调的意味吗？

"It is the individual visits [which are the most important]." Replace 'it' with the phrase in brackets... "[(Those) which are the most important] are the individual visits." You are right that in many cases it is preferred to use 'that' when presenting identifying or new information about a noun, and use ", which" when presenting implied information. "The cat that visited yesterday was orange." "The dolphin, which is a mammal, breathes air." In this case the rule is not really in effect, because it is a somewhat different construction. The phrase actually replaces 'it'. Using 'that' could sound like you are going to continue the statement: "It is the individual visits that are the most important which we will discuss during this meeting." In truth, though, you could use 'which' or 'that' just fine here; it is your choice. Notice 'which/that' is not always necessary: "It's nice the way the yard looks." Maybe also check out Philosophy of Grammar, Grammatical Types (Jespersen): https://books.google.com/books?id=_7CERdwh78QC&lpg;=PA24&ots;=OWI8bFZ05v&dq;=philosophy%20of%20grammar%20%22grammatical%20types%22&pg;=PA26#v=onepage&q;=philosophy%20of%20grammar%20%22grammatical%20types%22&f;=false