Default settings in a computer, are the settings the computer had when you bought it. Often we need to change these settings. Sometimes, if we change the settings too often, the default settings can be used as a reference. I hope that answers your question.

As Alan suggests, they are the settings used when you do not know the details or have not personalised anything for yourself. Sometimes the default settings work well, sometimes they are just the starting values & you need to change things to make things work.