As title， is there a word in Mandarin Chinese to mean "mostly" ? For example, I am going to tell my friend who wrote a letter to me that "I mostly got what you man", is it appropriate to say "大多" that I found in dictionary in this case for "mostly"?
Of course, but that just depends on different kinds of senarios:
1) “相当，非常……”（程度副词）: adverb something related to the depth:
I mostly love peace (我**非常**热爱和平)
2）It's mostly bad=It's quite bad=It's awfully bad 糟糕**透**了！（implementation to describe "bad"'s depth in Chinese, but that's used as an adverb in English).
3）包含很大程度上的可能性，肯定： Mostly speaking, It's almost of no hope to save people who sunk in the sea disaster of Korea.（可以这么说，救援韩国海难沉入水的群众希望破灭了）。
You can use the phrase 大部分, which means "most of it". "I mostly get what you mean." can be translated to 你说的话我大部分都懂.