Arduino Boards are microcontroller platforms that allow you to connect sensors and inputs then be able to trigger outputs to devices such as LED’s, Motors, and even network communication.
Arduino’s are great devices to start learning basic technology on because they are not computers and so there are no operating systems, or complicated configurations to learn before students start building projects. For the Arduino you connect the electrical devices to the board, and then write code that tells the Arduino what to do. Basically the Arduino runs a continuous loop with the code where it constantly reads from the inputs that are connected, and then turns connected outputs on or off based on the code.
This is what a very basic Sketch to make an LED Blink for an Arduino looks like: