Gmail predicts which emails might be of high importance to you so it marks them as important with a yellow icon. If your inbox ends up full of so called important emails, this is how you disable this feature.
If you don't want Google to store your chat history and the Google chat history of the person you are chatting with, you can take the chat off the record and this is how you do that.
If you were wondering where in Gmail can you actually see all the contacts you have stored with the help of your Android phone or any other way, this is how you get to see them.
