Santideva's Bodhicaryavatara is among the most important works in the Mahayana Buddhist canon. The title translates as the Way of the Bodhisattva. As such, it provides a concise presentation of the six perfections that are characteristic of the bodhisattva: generosity, morality, forebearance, vigor, concentration, and wisdom.
The work is composed of 912 verses divide into ten chapters. Composed in the 8th century, the Bodhicaryavatara played an important role in the transmission of Buddhism to Tibet and has remained important to Tibetan Buddhism. The Dalai Lama is among its admirers and Mahayana monks continue to memorize it today.
Its verses are generally clear and direct; however, chapter nine -- it's most celebrated chapter -- is quite difficult. Chapter nine address the perfection of wisdom as it is understood in the Madhyamaka tradition. Briefly, wisdom is coming to know that ultimate reality is "empty." This is the conclusion that is reached when all other metaphysical views about reality are refuted. For the Madhyamika, ultimate reality is neither one nor many, neither static nor changing, neither conscious nor unconscious. No analytic, discursive description of it is true. Language is only able to articulate conventional truth and at most can point in the direction of the approximate ultimate truth.
Chapter nine and the work in general owe much to the most important work in the Mahayana/Madhyamaka tradition, the Prajnaparamita. While the Prajnaparamita is far longer (100,000 verses in its longest form), it communicates the difficult concept of emptiness much more effectively. The Bodhicaryavatara is certainly worth reading, but other secondary material will make understanding its central ideas much easier.