It's explained in the sentence, isn't it? The sublittoral zone is below the high water line (sub- ; below) and the supralittoral zone is above the high water line (supra- ; above/beyond). The high water line being the farthest point up the shore the water reaches at high tide or due to general wave action in lakes and things that don't experience tides. So the sublittoral zone is between the high tide mark and the edge of the water and the supralittoral zone is above the high tide mark, but still near enough to the water body to be considered part of the 'shore'.
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timeasmymeasure
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Date: 2012-05-01 05:02 am (UTC)Does that make it clearer?