"汗 (hàn)" is the Chinese character for perspiration, or the sweat people produce when playing sports, or when faced with other intense or strenuous conditions. On the Internet, the word has come to indicate that one has nothing to say on an issue or conversation. In this way, it translates roughly into "I'm without words," or simply "I have nothing to add."
When chatting online with friends, it's actually quite common to see "汗 (hàn)," so much so in fact, that it's become a widely used expressi0n in our daily non-Internet based lives as well.
When you witness someone do something unusual, or extraordinary, you can express your astonishment by uttering "汗(hàn)." This usage of the word actually originated from a cartoon.
Finally, "汗 (hàn)" is a verb used to show emotions of embarrassment or shame.
Let's review the three common meanings of "汗 (hàn)."
First, to express a sense of surprise or shock when something exceeds expectations. It's English equivalent is along the lines of "wow," "holy cow," or "oh my!"
Hàn, nǐ jìngrán chī wán le zhème dà yí ge hànbǎo!
汗， 你 竟然 吃 完 了 这么 大 一个 汉堡！
Wow…you actually managed to finish that huge hamburger!
Second, to express a sense of embarrassment.
Hàn a, wǒ jìngrán bǎ yīfu chuān fǎn le.
汗 啊，我 竟然 把 衣服 穿 反 了。
How embarrassing, I had my clothes on backwards.
Last, to express being at a loss for words, or not having any reaction.
Hàn, wǒ zhēn bu zhīdao shuō shénme hǎo le.
汗， 我 真 不 知道 说 什么 好 了。
Oh…I really don't know what to say.