https://www.gov.uk/power-of-attorney
There are two types of POA, Health & Welfare & Property & Financial as Flonska states. POA’s kick in when someone is no longer capable of dealing with their affairs and cease when the person dies.
So if you are husband and wife for example it would be a good idea to implement as you never know what is going to happen in the future especially if you own property together. Or if you have children make your children POA and then both of you are covered should it be needed.
My brother & I obtained POA for our mother for both health & finances using the government website. Very straightforward no solicitor involved just the fees that need to be paid.