這句中，that 的意思是甚麼？

第一要注意的是 now that 是兩字一起的，例句中有兩個分句（clause）
I am understanding physics much better （main clause）
now that Mr Davies is teaching us. （adverb clause）

不錯，now that 意思是 for the reason that，是有「因為」的意思。不過 now that 同時表示分句所表述的情況是跟以前不同的。 now that 指出處境改變有新的因素令結果（main clause 所述）出現。

now that Mr Davies is teaching us 表示不久前教我們的不是 Mr Davies。

以前答過類似問題，再列舉用過的例句。
(http://hk.knowledge.yahoo.com/question/question?qid=7012083000938)

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.

Now that my daughter is big enough to take care of herself, I do not need a full-time maid.
Again, this sentence tells of a change of condition, i.e., my daughter has grown up.

lol@ I AM UNDERSTANDING physics much better...

May I also say garlic IS BELONGING to Yahoo Knowledge?

這個 that 是同前面的 now 合起來用，意思大概是「既然」。

that 的意思是同時

that=pronoun=the person mentioned=Mr.Daviies.

maybe is 因為.
因為佢應該是解釋後面果句