這句的 that 指甚麼？

Now that (two words together) is a conjunction meaning "for the reason that".
"Now that" is used as an adverb clause of reason to indicate a situation that arises because something has changed.

Do not confuse now that with that used in a noun clause.

He has more time now that he is retired.
In this sentence, that he is retired is the reason for his having more time. "Now that" tells us that he was still working some time ago.

Another example:
Now that my daughter is big enough to take care of herself, I do not need a full-time maid.
(http://hk.knowledge.yahoo.com/question/question?qid=7011080600192)

Again, this sentence tells a change of condition, i.e., my daughter has grown up.

這句的that是連詞，要來帶出＂原因＂

Longman Dictionary of Comtemporary English就對that作為連接詞，有這樣的解釋：
used after verbs, nouns, and adjectives to introduce a clause which shows what someone says or thinks, or states a fact or reason
它舉出的例子是：
He might have left the money for the simple reason that he didn't know it was there.

而yahoo字典，也有這樣的解釋：
(引導副詞子句, 表示原因或理由)因為, 由於We are glad that we have accomplished another arduous task. 我們都感到高興因為又完成了一項艱鉅的任務。

希望幫到你！

"that"=a conjunction,used to introduce a new part of the sentence:-He has more time-----He is retired.
He has more time now (that) he is retired.
(that) shows how much time he has since retired.