What does ‘buying on the MLS’ mean exactly?

[Felix Bird]

I am a recent newbie to Bigger pockets and a total convert! I am so impressed with the real estate rookie podcast it is really such a great way to learn about real estate investing in such an inspiring and interesting way. 
My question is this: What does it mean exactly when guests on the podcast say they bought their property on the MLS? I have assumed it means that they bought a property that was listed for sale online on say Zillow, Trulia, Redfin etc. which I assume all source their database of properties from the MLS? Ie. That they bought their property the way a regular homebuyers do. Or am I perhaps mistaken and is the MLS something different?
Best regards,

Felix