Passive income is income that you make without doing much work for the money.
Examples of passive income are, Crowdfunded Real Estate Investments, High Yield Savings accounts where your savings is earning money through interest over the years, renting out a room in your house, blogging, affiliate marketing etc.
A lot of people earn an eventual passive income through blogging after they initially work on their blogs in the beginning for at least 5 years and then after 5 years of steadily building up content on their blog and getting traffic to that blog the money eventually starts coming in even while you sleep.
Ways to earn a passive income through blogging are, affiliate programs, Adsense, Media.net and even direct ad sales when you have enough website traffic.
You could also sell an E book or other course on your blog and earn a good amount of money through blogging while you sleep or are away from your blog or computer.
It just takes hard work in the beginning but it can be done.