It means places where the barrier, or ceiling, between earth and heaven is thin. Where you sense heaven breaking through and being close.
The Bible says it is a terrible thing to be in the hands of the living God – or to be in his proximity. There’s two ways to read terrible, just as there is two ways to read fear.
In the Bible, it usually is the meaning that is BOTH! Like it being terrible, in the sense of riding the Big One, in the grip of circumstances over which you have no control, but knowing you are safe, and enjoying the perceived danger. I guess, if you have an in-built fear of heights, or previous bad experience of roller-coasters, that may so dominate that you don’t get the thrill of terrible. You need a different example, but the principle is the same!
A thin place should not be a haven of tranquility, but a source of peaceful oil onto turbulent waters. A place of invigorating refreshment, not a short-term shelter from the storm. A place of fruitful labour rather than a luxury spa.
Thin places. Find and visit them.