Paym is a service offered by a number of banks and building societies, which allows money to be sent simply using the mobile number of your contacts rather than the need for their account number and sort code.
If someone tries to send you funds through Paym and you have not registered, you will receive a text like "Someone has tried to send you money via Paym, the UK's mobile payments service. To learn more about Paym, contact your bank or building society. Opt-out? Reply STOP."
This means the payment has been rejected and won't be made to you.
If you would like to register so you can benefit from Paym in the future, you can through the mobile app.
For more information why not visit the Paym website.