It's pointing out that there is a water layer at times b/w the land and ice and that sometimes it can be pretty deep.
“It’s pointing out that there is a water layer at times b/w the land and ice and that sometimes it can be pretty deep.”
The issue is further complicated because the weight of the ice pushes the land mass down. That may mean the land is below sea level which would reduce the amount of sea level rise when the ice melts. However, some think that if the ice melts the land mass will rise and that would increase the sea level. All this is hypothetical anyway since the ice isnt melting.