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 matrix representation.

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

For state vectors $\lvert \psi\rangle$, it's applied $X \lvert$

On the Bloch sphere, it rotates the state by $\pi$ radians (180 degrees) around the X-axis. The state $\lvert 0\rangle$ is associated with the north pole

If you apply the gate twice, you will flip the state twice, meaning you $$X^2 = I$$