When you work with an Arduino on a project and you need to control a digital output, they can have one of two states. The output can either be high or low.
When the output is pressed high, the entire supply voltage is fed to the pin and it can then be used to power an LED or turn on a device depending on the project. This configuration is called purchasing where the current source is Arduino. In this way the current goes from the power source, enters the Arduino and then goes into the load.