If I've understood it correctly, it's the idea that the way our mind works is severely constrained by our physical form. For example, one of my pet hypotheses is that, since we are bipeds that grow up vertically, we're conditioned to think that more important things are in a vertically higher position than less important things (our language is littered with such metaphors: superior, inferior, exalted, debased, etc.). It shouldn't be immediately obvious that things farther from the ground have greater value, but I've found it difficult to show to other people that vertical metaphors are metaphors, and that we'd use different ones if our bodies were different.