A power of attorney (POA) is a legal document where you (the "principal") authorize another person (the "agent" or "attorney-in-fact") to act on your behalf in financial, legal, or medical matters. It doesn't transfer ownership of anything, it just lets the agent make decisions and sign documents as if they were you, within whatever scope the document grants.
The scope varies a lot depending on the type of POA you set up. A general power of attorney gives broad authority covering most financial and legal matters: banking, paying bills, managing property, filing taxes, handling investments, even entering contracts on your behalf. A limited (or "special") power of attorney restricts the agent to specific tasks, like selling one particular property or handling a single transaction while you're unavailable. A medical power of attorney (sometimes called a healthcare proxy) is separate and only covers medical treatment decisions if you're unable to make them yourself.
The "just in case" framing usually refers to a durable power of attorney, which is the version that stays in effect even if you become mentally incapacitated, due to injury, illness, or dementia, for example. A regular (non-durable) POA automatically ends if you lose capacity, which defeats the purpose for most estate-planning use cases. Durable POAs are what let a spouse or adult child step in to manage your bills and affairs without going to court for a conservatorship if something happens to you.
A POA can also be set up as "springing," meaning it only becomes active upon a triggering event (typically a doctor certifying incapacity), rather than being effective the moment it's signed.
Importantly, a power of attorney automatically ends when the principal dies. At that point, an executor (named in a will) or trustee (if assets are in a trust) takes over instead. A POA also can be revoked by the principal at any time while they're competent to do so, and it ends if a court determines the agent is abusing the role.
Because the authority granted can be extensive, the choice of agent matters a great deal. This is a role that requires real trust, since a poorly chosen or dishonest agent can do significant financial damage. Many people limit scope deliberately, or name co-agents who must act together, specifically to add a layer of oversight.