The Arduino is an open-source electronics prototyping platform. It is low cost and it is great for the creation of interesting projects. The Arduino contains an microcontroller to which one can apply logic to control input and output using different sensors and components.
If you have ever heard of, or played with, a Raspberry Pi, this Arduino project is similar with the exception that the Raspberry Pi is more of small computer that can run multiple programs at the time while the Arduino is more of a prototyping microcontroller; better for running one program at the time.
There are several versions of Arduino for different purposes. If you’d like to learn more about Arduino, I suggest you visit their website, which is great and has lots of info to get started.
Also, there are great starter kits on Amazon, here’s a link to one Arduino Mega Kit, but there are many other options.
Some Things I’ve Done with Arduino (Basic projects)
These are only some examples and basic ones, but there are way cooler things you can do with it. You can check this page for project ideas and see what other people are doing with it: https://www.hackster.io/arduino/projects