In general, a default is the option offered to you, which is either recommended or safe to choose if you're not sure. In computer operating systems and programs, the default option is chosen if you press Enter without making a selection. The default may also be the position of a toggle selector if you don't interact with it, or data automatically entered in an unedited text box.
The following are examples of defaults you may encounter when using your operating system.
The default location is where a program defaults to first when saving or installing 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 in Windows 8 or earlier and Microsoft Edge in Windows 10. With macOS on Apple computers, the default browser is Safari.