Congratulations! You own a (very small) slice of Apple.
As a stockholder, you have a vote on important decisions that the company makes. Each year Apple has a stockholder meeting in Cupertino that you are invited to. If you are unable to attend and vote, you can vote by proxy, which simply means that you register your vote before the meeting. You just missed this year's meeting, which was held on February 26, 2016. They elected people to the board of directors, chose an accounting firm, and voted on some other proposals. Votes are based on the number of shares you own; since you only own one share, your vote is very small compared to some of the other stockholders.
Besides voting, you are entitled to receive profit from the company, if the company chooses to pay this out in the form of dividends. Apple's dividend for the last several quarters has been $0.52 per share, which means that you will likely receive 4 small checks from Apple each year.
The value of the share of stock that you have changes daily. Today, it is worth about $100. You can sell this stock whenever you like; however, since you have a paper certificate, in order to sell this stock on the stock market, you would need to give your certificate to a stock broker before they can sell it for you. The broker will charge a fee to sell it for you.
Apple has a website for stockholders at investor.apple.com with some more information about owning Apple stock. One of the things you'll find here is information on how to update your contact information, which you will want to do if you move, so that Apple can continue to send you your proxy materials and dividend checks.