"CellSplit" is a front end token that splits a cell into two or three cells at the current selection.
This token is equivalent to the menu command Cell Divide Cell.
An insertion point divides the cell in two. A text selection divides the cell into three cells: everything before the beginning of the selection, the selection itself, and everything after the end of the selection.
To execute a front end token, you typically evaluate a command of the form FrontEndExecute[FrontEndToken[token]].