A power of attorney is a legal document giving someone else authority to act in your behalf.  This can be limited, for one transaction or for a set period of time.  It can also be durable, meaning that it stands until it is revoked or superseded.  Most estate plans include a durable power of attorney.  This can be used if you become incapacitated.  At that point, the person with power of attorney can step in and act on your behalf.  They are usually called an agent or attorney-in-fact.  There is usually a primary (typically your spouse) and then a few back-ups.  I do not generally recommend having co-agents.  Your agent will typically send your power of attorney to a bank or financial institution to access your accounts or sign for things.  If the power of attorney is outside of the normal parameters of what that employee typically sees, they will often kick it over to the black hole of the “legal department” where it can sit for a while until someone signs off on it.

You are giving the agent a lot of access and control with a power of attorney, so choose someone you trust.  However, they have a fiduciary duty to do what is in your best interest.  If they do what is in their best interest and breach that duty, you or your heirs could have recourse against them.