不好意思(bù hǎo yìsi). This expression is generally used to make an apology, which indicates that the speaker is sorry for what has happened or what is going to happen. It can also mean being shy, feeling embarrassed, and polite (equal to "excuse me" in English).

Examples:
A: Bù hǎo yìsi, wǒ chí dào le.
A：不 好 意思，我 迟 到 了。
A: Sorry, I'm late.
B: Méi ɡuānxi.
B：没 关系。
B: It doesn't matter.

A: Bù hǎo yìsi, Wǒ zhōngwén shuō de tài chà le.
A: 不 好 意思，我 中文 说 得 太 差 了。
A: I am sorry, my Chinese is very bad.
B: Méi shì, mànmàn lái.
B: 没 事， 慢慢 来。
B: Take it easy, and be patient.