If you are making a post in an existing forum, you often want to reply to a previous posters question. If that persons post is not the last post made, it can be difficult to let people know you are replying to that particular poster.
Example:
User 1: Hi I'm Dave.... anyone else work in the Healthcare industry?
User 2: Hi I'm Mike... I'm in the Aviation industry...
User 3 : Hi I'm Jim, I'm in the Food industry
Me : Welcome to the community!
User 4: I'm from the Healthcare industry too, User 1.
User 4 can specifically make his reply to "Dave" by using the quote feature.
Reply to the Dave's post by clicking on the 
button on his post.
You will be presented with the reply page. Above the message box, you should see the previous post in a grey box.
You can quote all of this post by clicking the 
button. This will add some code into your reply. Alternatively, you can highlight a specific sentence or word by highlighting the desired text. Hitting "quote" will only quote the highlighted text.
The following post shows what quoting a sentence looks like.