그

That

그 means "that". It can be a noun or an adjective.

• "그는 안 했다." He didn't do it. ("that" is the thing that didn't do it, so is a noun.)
• "그 사람 아니다." It's not that person. ("that" describes "person" and so is an adjective.)

그, as seen in the example above, is often used to refer to people, usually the third party "he" or "she". It is also used for non-human things as "it".

Ahh...

그 also is used, sometimes, as we use "ahhh..." or "ummm...", a word to say when you are thinking.