T = Technology
P = Pedagogy
K = Knowledge
In a nutshell, TPACK is a synthesis of the idea that high-quality instruction integrates technology with awareness of developmental appropriateness (pedagogy) and content knowledge, or expertise of the teacher. It is important for many reasons, the most significant being that every student learns differently. Through TPACK, teachers can frame the context around pedagogical parameters and the content being taught, and integrate technologies that best target his or her students.
For more information, check out these videos!
(image above is rights free from http://www.tpack.org)