There are many ways to say “sorry” in Mandarin Chinese, but one of the most common and versatile phrases is ►duì bu qǐ. It means "sorry" in the sense that you have wronged someone and want to apologise. The phrase consists of three Chinese characters: 对不起 (對不起 in traditional Chinese):

对 (duì) in this case means" to face", but can in other situations mean many other things, such as "correct" or "to".

不 (bù), is a negative particle that can be translated as "no" or "not".
起 (qǐ), literally means "to rise”, but is often used in an extended meaning "to be able to".
If you put these together, you get something like "unable to face", which is the feeling you have when you have wronged someone. This phrase in Chinese can function as a standalone way of saying "sorry", but it can also be used as a verb, so you can say:

我对不起你

wǒ duìbuqǐ nǐ

I have wronged you.

Let's look at a few more examples. As you will see, the thing you have done to wrong the other needn't be as serious as all that, this is often just a way of being polite, just like saying "sorry" is in English.

►Duì bu qǐ, wǒ gāi zǒu le.
對不起, 我該走了｡
对不起, 我该走了｡
Sorry, I have to go now.

►Rú guǒ wǒ shuō duì bu qǐ, nǐ shì fǒu jiù huì yuán liàng wǒ?
如果我說對不起, 你是否就會原諒我?
如果我说对不起, 你是否就会原谅我?
If I say I’m sorry, will you be able to forgive me?
It should be mentioned that there are other ways of interpreting or breaking down this phrase.

You could also think of it as 对 meaning "to treat" or "correct", which would give the sense that you have not treated someone the right way or that you have done them wrong. For practical purposes, it matters little which you use; pick whichever explanation you find easiest to memorise.