That is the main ground for the bike. It bolts to the engine casing behind the starter - you can see it from the right side of the bike easily. If that isn't secured, your electrical system will work about as well as a brick floats. Definitely repair that as Jerry suggests, or make a new bigger ground cable, which is what I did.
I also noticed somebody removed the 10A fuse from the system too and it looks like they just soldered and heat shrank the wires together after cutting out the fuse holder. That is no bueno.
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