Raspberry Pi, along with Arduino, are the brains of Maker projects!
Raspberry Pis (aka RPi, aka Pi) are small, cheap Linux computers designed to make it easy to develop projects that would otherwise required an expensive dedicated computer.
The Pi has HDMI and composite outputs for plugging in to a monitor, and usb ports for keyboard, mouse, and other peripherals. Think of it like a regular Linux computer, just REALLY small. It's often used in conjunction with Arduino to help with network communication.
Check out the Maker Hub's workshop pages for upcoming Raspberry Pi classes!