Explanation
This formula uses structured referencing, a syntax that allows table parts to be referred to by name. When a table is referred to by the name only, Excel returns a reference to the data region of the table only. In this case, the entire table range is B4:F104 so Table1 returns the range B5:F105 to the COLUMNS function.
=COLUMNS(Table1)
=COLUMNS(B5:F105)
COLUMNS then returns a final result of 5, since there are 5 columns in the table.