A Lasting Power of Attorney (LPA) is a legal document that lets a person (the 'donor') appoint one or more trusted people (the 'attorneys') to make decisions on their behalf if they later lose the mental capacity to do so themselves. It is one of the most important legal documents most people never get around to making.
An LPA can only be made while the person still has mental capacity. If someone loses capacity (through dementia, illness or accident) without an LPA in place, their family must apply to the Court of Protection — a far slower, more expensive and more stressful process. Setting up an LPA while well is straightforward and inexpensive (registered through the Office of the Public Guardian in England and Wales; different systems apply in Scotland and Northern Ireland).
This is general information, not legal advice. Many people set up an LPA themselves via gov.uk, but a solicitor can help with more complex situations.