The IS functions answer TRUE or FALSE about what a cell holds:
ISBLANK(C2): the cell holds nothing at all, not even a formula.ISNUMBER(C2): it holds a number (dates count, because dates are numbers).ISTEXT(C2): it holds text, including digits stored as text.ISERROR(C2): it holds any error, such as #N/A or #DIV/0!.
Two traps. A formula that returns "" looks empty but isn't blank, so ISBLANK says FALSE; test C2="" instead. And a number stored as text fails ISNUMBER, which is a quick way to find the cells that break your SUMs.
They're most useful inside IF: =IF(ISNUMBER(C2),C2*2,"Check").