In power exchange, one person chooses to offer some part of their power to another (within limits such as duration, safety, and so on), and the other accepts the responsibility of taking on that authority.
There are many forms of power exchange. For example, in bondage, one person (the person who likes being tied up) gives power over their body to another (the person doing the tying up).
Informed consent is important for both sides. “Informed” means that the person fully understands what they’re consenting to. For example, if someone wants to be tied up, but doesn’t disclose that they’re prone to seizures, that’s not informed consent for the person who says “yes” to tying them up.
Power exchange is also called “BDSM”, but I don’t like acronyms very much, and people often have a lot of preconceived concepts about what “BDSM” is which doesn’t cover the full range of possibilities. So rather than say “BDSM” is about power exchange, I’d rather just say power exchange.