What Is _ In Maths?

It depends on where you see it.

In second grade, ___ represents an unknown quantity, as in
___ + 3 = 5.
It is "the blank" you are to fill in when instructed to "fill in the blank."

In later grades, ___ separates the numerator from the denominator of fractions. In text it is represented by a slash (/) as in
3/4.
In this context, it means "divided by", as in "3 divided by 4".

When a numerator or denominator is not a single variable or constant, it must be enclosed in parentheses when writing the expression in text form, as in
(x+1)/(3x)
If this were written without parentheses, it would be interpreted as
x+1/3x = x(1 1/3) = (4x)/3
The two expressions, (x+1)/(3x) and (4x)/3, are not the same.