The IF function has three arguments:
=IF(condition to check, what to return if condition is true, what to return if condition is false)
The third (last) argument is optional. If you leave it off, and the condition is false, it will return the word "False". Most people do not want to see this.
By using "", we are literally telling it to return an empty string (nothing) if false.
Regarding dates, dates are actually stored in Excel as numbers, specifically the number of days since 1/0/1900.
You can easily see this by entering any valid date in Excel, and then changing the format of that cell to "General" or "Number". You will see the large number that Excel stores the date as.
So dates in Excel really are just numbers with a special date format applied to them.
Since dates really are numbers, you can just check to see if the value is greater than 0, i.e.
Excel Formula:
=IF('All Wolves'!J15>0,"x","")