X-gate (or Pauli-X gate, or Quantum NOT gate) is a quantum gate that flips $\lvert 0\rangle$ to $\lvert 1\rangle$, and $\lvert 1\rangle$ to $\lvert 0\rangle$, which is why it's called the quantum equivalent of NOT. It has the following 2×2 matrix representation.

$$ X = \begin{pmatrix}0 & 1\\ 1 & 0\end{pmatrix}$$

On the Bloch sphere, it rotates the qubit state by $\pi$ radians i.e. 180 degrees around the X-axis. The state $\lvert 0\rangle$ is associated with the north pole, so it flips it to the south pole. The state $\lvert 1\rangle$ is associated with the south pole, so it flips it to the north pole.

If you apply the gate twice, you will flip the state twice, meaning you go back to where you started. This means. This property is called the “involution” property of X-gate. Other Pauli gates also have this property $$X^2 = I$$