Passive income is income that requires little to no effort to earn and maintain. It requires some efforts during the initial setup but generates steady cashflow thereafter. It is where you are still getting paid for work done a long time ago.

This could be royalties from pieces of art, books, etc. or rent from a house, dividends from stocks, a business that does not require your supervision once established, websites, blogs and so on.

The reason behind earning a passive income could be financial independence where you slowly build passive income sources where one day the total passive income generated is equal to or greater than your current income from job/business. So that you can escape the 9 to 5 and do things you are passionate about with complete independence. Otherwise, you may just want to earn some extra cash from activities you are passionate about and would have done anyways without any monetary expectations such as photography, blogging, writing, etc.