In general, a default is what the computer, operating system, or program has set for the user based off what it believes is the best option. If the user wants a different setting, the default can be changed to a user setting. Below are some different examples of how default is used with computers.
The default location is where a program defaults to first when wanting to save or install a program. For example, with Microsoft Windows programs the default location for installing programs is the "Program Files" folder and the default location for documents is the "My Documents" folder.
When talking about settings of a program, the default settings are the settings chosen by the software developer and not the user. For example, when opening Microsoft Word the default font setting is a font size of 11.
With a program, the default program is what program opens when opening a certain file type, more commonly known as the file association. For example, the file extension .XLS may be defaulted to Microsoft Excel, and when opening the file it opens in Microsoft Excel. When installing a new program, it is not uncommon for the default program file association to change unless the program gives you the option of not changing the settings.
The default browser is the Internet browser the computer has chosen to browse the Internet. For example, with Microsoft Windows the default browser is Internet Explorer and with the Apple computers its Safari.