Blockchain is the technology that cryptocurrency runs on.

You know Visa and Paypal and Mastercard? Those systems run on big databases held by those companies. If those companies fail then everyone loses their money (or the debt). Blockchain however, runs on thousands of people's computers, and those people get paid a little slice for helping out.

Steem uses blockchain technology... so it runs on the top 20 witnesses computers... so if any of those 20 people have a computer problem, the next person just steps right in.