It's crucial to encrypt your Bitcoin wallet in order to prevent theft. This can be done by clicking on "settings" in the Bitcoin wallet menu and then choosing "encrypt wallet". Essentially when you encrypt your wallet you set a password. You can still see your wallet balance and receive Bitcoins without the password, but in order to send Bitcoin you need to enter the password. Hackers have designed many programs which steal wallet files off people's computers, and if your wallet is unencrypted and you download such a program your Bitcoins will quickly disappear. Once Bitcoins are sent to a thief it is impossible to get them back, so it is important to prevent the theft in the first place.