The will to power is a central idea in Nietzsche's philosophy. It is, to him, the main force that drives the ways in which human beings behave.
Nietzsche felt that all human beings (and indeed all beings) were constantly striving to increase their domain. They were trying their best to exert their control farther and farther. This desire to expand one's influence is the will to power.
Since you placed your question in the History section, let me address this idea in that context a little. This idea clearly impacted Hitler in his thinking. The idea that all people and, by extension, all countries wanted to extend their power struck a chord with Hitler. It helped to influence his thinking and to cause him to be as aggressive as he was.