A standard set of rules for collecting and sending events directly to Google Analytics servers.
Measurement Protocol lets you send data to Analytics from internet-connected devices like a kiosk or point-of-sale system that complement your website or app. Measurement Protocol is a different method of sending data than gtag, Google Tag Manager, and the Firebase SDKs. While implementing these methods automatically sends events to Analytics, with Measurement Protocol you have to manually program events. In addition to sending new events, Measurement Protocol also lets you add information to events you've previously collected via gtag, Google Tag Manager, and the Firebase SDKs.
Learn more about generating API secrets for Measurement Protocol.
Read about the Measurement Protocol on Google Developers.