A default is the initial state of a "thing". With software the defaults are the preference settings a program has before you change them to suit your particular tastes.
Default settings are important as they can easily become the status-quo. Document templates for programs such as Microsoft Word are the starting point for less adventurous/experienced computer users. In the case of Word, this includes using American English for spell-checking, leading to the unintentional substitution of commonly used words, e.g. "color", "catalog" and "favorite".