Lego mindstorms ev3 spinner factory problem
My Blocks can have input and output parameters. 

What you're seeing is an input parameter. 

This particular My Block has as input parameter the color the factory bridge needs to drive to.

If you create a My Block from a selection of blocks and there are already data wires going in or out of the selection, input and output parameters are created automatically.

You can also add or edit input or output parameters in the My Block Builder dialog.

Perhaps this tutorial helps:
http://stemrobotics.cs.pdx.edu/sites/default/files/MyBlocks%20%281%29.pdf