Power of Attorney (PoA), also known as a letter of attorney, is a document you can use to give someone legal rights to act on your behalf.
This document is usually signed by the party giving the responsibilities to the other party. So in case you are looking to file a trademark application with an IP Office and need someone to act as your legal representative, it would be you who's signing the Power of Attorney. Depending on the specific form, the Power of Attorney can also be signed by both parties present, meaning the party which is authorising and the party which will be using the competencies gained (in most cases, this would be an attorney or law firm).
A power of attorney can be granted digitally in most cases. Usually, you will receive a form with pre-filled information from your legal representative, which you then have to sign. This can be done remotely, and printed, signed and rescanned documents are also accepted. The document is valid for a specified stated time. Some jurisdictions require submitting notarised or legalised Power of Attorney, but it's rare. Your legal representative should let you know if it applies to your case.