"EvaluateCells" is a front end token that sends the selected cells to the kernel for evaluation.
This token is equivalent to the menu command Kernel Evaluation Evaluate Cells.
Only cells with the option EvaluatableTrue (normally cells in the Input style) will be evaluated. All other cells will be ignored. See Evaluate Cells for more information.
To execute a front end token, you typically evaluate a command of the form FrontEndExecute[FrontEndToken[token]].