Need Help with log Equation

So in my university Astronomy course I need to use log equations, I have never done any before and do not know what log is, although I am expected to...the equation I need to solve is m - M = 5 x log(d/10) ...d is 1.34 for the specific question...I don't understand what I am supposed to be solving for (is it either the m or the M??) and I don't know what that 5 is doing there (I have no breakdown of what each part of the equation means). Please explain what log is and how to solve the equation, thank you!