Parentheses notation in number theory

Question

I'm looking at the following problem.

$p$ is a prime number. For $a,b,c \in \mathbb{N}$ such that $ab^2 \equiv c^2$ (mod $p$). If $(\frac{a}{p}) = -1$, then $b^2 \equiv c^2 $(mod $p^2$).

The solution says that $p$ does not divide $a$ because $(\frac{a}{p}) = -1$. What is $()$ suppose to mean?

Thank you.

Answer 1

It’s the Legendre symbol. When it’s $-1$, the top number is not a quadratic residue (perfect square) modulo the lower number.