Cable Plug Tool for Di2
Shimano's TL-EW02 Cable Plug Tool is one of those tools that you only use when building or maintaining your bike. It allows you to securely and properly connect and disconnect EW-SD50 electric wires, connecting your Di2 components to each other. Plugging in Di2 connectors correctly requires more force than you'd expect and a loose connector can cause all kinds if weird issues if you're unlucky.
When you plug in components correctly you should feel and hear a click (or 'pop').
Using the tool
Both ends of the cable plug tool are used when connecting components. One end is used almost exclusively for connecting wires in hard-to-reach places such as the front derailleur, while the other end is used whenever you've got a bit more room.
TL-EW02 vs TL-EW300
Shimano's newer electronic groups do not use EW-SD50 wires, but EW-SD300 instead. These newer wires are smaller, meaning that you cannot use the TL-EW02 to plug them in / unplug them. Instead you'll have to use the TL-EW300.