You get this notification automatically when you've asked a question without accepting an answer.
If someone has answered your question correctly, in a way that solves the problem you had when you asked it, you're encouraged to accept it. However, this is not mandatory, and if nobody has answered your question to your satisfaction, there is no obligation to accept an answer anyway.
If you haven't received any satisfactory answers, and you have more rep than you know what to do with, you can also start a bounty on your question by offering a rep incentive to anyone who answers it correctly. However, note that by doing so, you lose that amount of rep whether or not somebody answers the question and receives it.
Really, you don't have to pay any attention to this automatic message, as long as you know how to accept answers (which evidently you do, since you have accepted answers on a couple of your questions). It's automatically generated by the system, and will go away in a month. Consider it a reminder to check through your question list and see if there are any accept-worthy answers you've forgotten to accept, not a decree telling you that you must accept more answers.