The mean longitude is the sum of the right ascension of the ascending node $\Omega$, the argument of periapsis $\omega$ and the mean anomaly $M$:
$L = \Omega + M + \omega$
As $\Omega$ is measured in the equatorial plane and $\omega$ and $M$ in the orbital plane, $L$ is a broken angle and represents an angle if (and only if) the orbit is circular and equatorial (i.e. no inclination)
For a satellite on a keplerian orbit, the mean anomaly defines the position along the orbit at a given epoch. It is for instance used in the two-line element set (TLE).
So the mean longitude is a combination of measures in the equatorial plane and the orbital plane and the mean anomaly is a measure in the orbital plane only.
Hope it will help.