The Shimano Di2 SM-BCR1 Power Cable is your Di2 battery's connection to the grid.
Whether you're buying the Shimano SM-BCR1 charger for your external battery or the Shimano SM-BCR2 charger for your internal battery so you can leave the battery on the bike, you need a power cable to get the charger attached to electricity. This is that cable.
There are choices because there are different plugs and power standards around the world. You might need one for home and a different one for travel.
The Shimano Di2 SM-BCR1 is the final step to charging your Di2 battery.