Maghrib is the fourth of the mandatory daily prayers in Islam. Maghrib begins after the sun has set. Maghrib time ends when night has fallen and the sky is dark, thus starting the time for the Isha prayer.
The Maghrib prayer consists of three obligatory prayers. Then two Sunnah (recommended) prayers are offered, followed by two Nafl prayers. The Nafl is an extra prayer, but not mandatory. It is better to read it though, then to not read it. Many people do pray them, unless they are short on time (which really isn't a suffice reason).
To learn more about what all this means, it might be helpful to read the writeup on Salat by robwicks.
The other four prayers during the day are Fajr, Zuhr, Asr, and Isha.