The CELL function can provide a wide range of information about cell properties. One property is called "protect" and indicates whether a cell is unlocked or locked. All cells start out "locked" in a new Excel workbook, but this setting has no effect until a worksheet is protected.
The CELL function returns either 1 or zero to indicate "on" or "off". In this case we are comparing the result to zero, so when CELL returns 0, the expression returns TRUE and the conditional formatting is triggered. When CELL returns 1, the expression returns FALSE and no conditional formatting is applied.