The volume at which music is performed is called Dynamics
The two most basic Dynamic terms are:
Piano, abbreviated p - which means "soft."
Forte, abbreviated f - which means "loud."
The term "Mezzo" is often used as a modifier for the dynamics listed above. Mezzo, abbreviated"m" means "Medium."
Therefore, if you see the marking "mp" you would interpret that dynamic marking as "mezzo piano" or medium soft.
Similarly, if you see the marking "mf" you would interpret that dynamic marking as "mezzo forte" or medium loud.
In addition to the modifier, "mezzo," often you will see a dynamic marking doubled. For example, "pp" or "ff" mean pianissimo (very soft) or fortissimo (very loud), respectively.
The most common dynamic markings then, from softest to loudest, are: