Scratch

Well, let me try to explain, though you can find a great tutorial here.

Remote sensor connections were added in Scratch 1.3, and were designed to help Scratch connect to the world outside. To do this, the following system was devised:
When you click on the drop-down of the ([ v] sensor value) block, you get the choice "turn on remote sensor connections", which turns on the feature. Now, Scratch listens on the localhost port 42001 for binary messages like "broadcast something", which it then evaluates. This allows for bidirectional sharing of broadcasts (you can send and receive broadcasts with an external program), Scratch global variables to be read from an external program, and "virtual sensors", which are like variables which are controlled by an external program, and accessed from the ([ v] sensor value).
The most common use of this would be the PicoBoard and Lego WeDo, which are real-world sensor and motor kits which connect to your computer via USB and use remote sensors to allow reading of data from the ([ v] sensor value) block.
Other uses include Enchanting, which connects Scratch to a Lego MindStorms NXT, the connection of Scratch to Scratch modifications, and Joystick support with Joytail.