It's an empty string.
It makes the cell look blank/empty--but it's not. It contains a formula.
Lots of people (too many!!) use:
=if(a1>7,"Too many"," ")
with the extra space character.
But this can cause trouble in cells that check the value in that cell.
=if(b1="","looks blank","doesn't look blank")
If b1 contained " ", then this formula would return the "doesn't look blank"
string.