From time to time, weaknesses are discovered in programmes running on your computer. These weaknesses can be exploited by virus writers and hackers to gain access to computers. As such, publishers will release 'patches' from time to time to correct these weaknesses.
To check for patches and updates you should visit the publisher's website, typically their 'Download' section. Generally, the latest versions of an operating system family (like Microsoft Windows) or browser (like Internet Explorer, Firefox etc) is the most secure.
Microsoft users can visit: http://windowsupdate.microsoft.com , which can automatically check what is required for both your operating system and browser and then download it at your request.