Someone asking what you hold about them. You have 30 days to respond.
A data subject access request is a person asking you what personal data you hold about them, why you hold it, who you share it with, and how long you keep it. They can also ask you to correct it, delete it, restrict its use, hand it over in a portable format, or object to a particular use.
You have 30 days from receiving the request to respond. The clock runs from when the request arrives, not from when you get round to it, and not from when you finish verifying identity, so verification needs to happen quickly.
A request does not have to use the words "data subject access request", cite the Act, or arrive on a form. An email saying "please send me everything you have about me" is a valid request, and so is one made verbally.
You may refuse a request in limited circumstances, but you must tell the person why and let them know they can complain to the Commissioner.