It means that this watch has been designed to resist water pressure up to 150 meters, or 500 feet. This is an unusually high level of water resistance for a non-dive watch, and I chose to do this precisely because of my experience with vintages. Too many good watches of the past have shown evidence of water damage. I wanted to use the best modern methods to ensure a dry watch under demanding conditions. :-)
15 ATM is a near dive rating, and it means you should be able to safely wear this watch in a pool or the shore (but not a hot tub, as hot water ruins gaskets) If you intend to submerge a water resistant watch, you must ensure the screwdown crown in screwed in all the way.