What does magnetic quantum number represents

It specifies the number of orbitals and their spatial arrangement within a subshell. The possible values of $m_l$ depend on the value of the azimuthal (or angular momentum) quantum number ($l$). For a given $l$, $m_l$ can take any integer value from $-l$ to $+l$, including zero.

For example:

• If $l=0$ (s subshell), $m_l = 0$, meaning there is only one s orbital, which is spherically symmetric.
• If $l=1$ (p subshell), $m_l = -1, 0, +1$, meaning there are three p orbitals ($p_x, p_y, p_z$), each oriented along a different axis.
• If $l=2$ (d subshell), $m_l = -2, -1, 0, +1, +2$, meaning there are five d orbitals with different spatial orientations.

The magnetic quantum number helps to explain the splitting of spectral lines in a magnetic field (Zeeman effect).