The MasterCard Internet Gateway Service (MIGS) is a payment gateway system that allows banks to accept "card not present transactions" such as online payments. It's used to interconnect online merchants to their acquiring banks through standards-compliant technology and API (Virtual Payment Client).
Some banks and services use MIGS to help integrate their Realtime Credit Card Payment Gateways, so if you're setting up this for your Neto Web Store you'll need to know the details for this. Once logged in to your MIGS accounts you can obtain the appropriate information to integrate your Neto Web store with your Payment Gateway provider. This sometimes varies from gateway to gateway.