As the name suggests, it's a message sent by your provider to let you know either that your existing contract is about to end, with a view to giving you a heads-up to start looking for a new deal.
Networks have been legally obliged to send these to customers since February 2020. They came into force after millions of customers were found to be overpaying for their existing service, due to networks' habit of dramatically hiking prices once customers' minimum contract terms have expired.