A Power of Attorney, or PoA, is a legal document through which you (the principal) authorise another person (the agent or attorney-holder) to act on your behalf in specified matters — this could include managing your bank accounts, buying or selling property, or handling investments, depending on how the document is drafted.
A PoA can be general, covering a broad range of matters, or specific, limited to a single defined transaction like selling one particular property, and it can also be temporary or valid until revoked. It's a useful tool when you're unable to personally handle your affairs, such as when living abroad or during a medical incapacity, but because it grants real legal authority over your finances, it should only ever be given to someone you deeply trust, and ideally drafted with proper legal guidance.