Sentence:  The region is split into two logical daughter regions. The split is not done physically at this point to make the splitting process fast. The split is maintained as a <WSD>split key</WSD>, which is a key in the middle of the key range maintained for the region. 
Meaning of the word marked with <WSD> tags:  A key value used to identify where to split the children of a parent node in a B-tree or similar data structure.