This work contains the backbone of Kant's most influential principle, the categorical imperative. This is essentially the notion that one should act with the intent that their action become a universal law. In essence, this is more to do with what cannot be a universal law, rather than what can. So therefore, false promises, loans that cannot be repaid, suicide, are all outlined by Kant as Maxim's that would destroy themselves.
Kant also gives a lengthy analysis of the importance of a good will, or good intent, as another cornerstone of his moral philosophy.
Immanuel Kant is perhaps the most influential philosopher of the modern era, and this work contains the principles from which many have borrowed, such as John Stewart Mill, and many have fiercely critiqued, such as Friedrich Nietzsche.
Despite its relatively short length, this is not an easy read as Kant elaborates upon the various points at some length, however it is nonetheless essential reading for any student of philosophy, or the social sciences.