United Payment Interface (UPI) is a mode of payment, that you can use to make immediate transfers at any time! You can find more details about this here.
There are a few payment service providers (PSP) authorized by the National Payment Corporation of India (NPCI). You can find their names and download their mobile apps here. You can set up your UPI and add a registered bank account. The UPI app will set up your Virtual Payment Address (VPA).
For e.g in BHIM, your default VPA will be <mobile number>@upi. If your mobile number is 123456789, then your VPA will be 123456789@upi.