Power is a Power category block. Connecting it to a Source block will power that source block, triggering it’s audio, and keeping it on until disconnected. This makes it useful if you want to create a synth drone, or if you have a sample (with a Sample, SampleGen or Recorder block) that you want to loop continuously, without using an interaction block to do it manually.
They’re also used to power Particle Emitters and Lasers, both of which need to be powered to operate. Blocks in the Power category can be combined to create interesting reactions, and self-playing environments. Read more on this subject on the Power page.
The Power block has no options, it is simply always on.