If I had to make a guess I'd say the wire runs the signal for an O2 sensor. O2 sensor wires are usually shielded wires (the unsheathed wire you see in the pic is wrapped around a sheathed wire) to prevent signal distortion. The yellow/black wire connected to the shield wire (bare wire) is most likely a ground (pretty common color scheme for ground) and it's being used to ground the shield wire. If this is the case, it's really not that uncommon to see a setup like this though not exactly kosher in some cases. The previous owner was probably trying to run an A/F sensor or nullify an O2 sensor. Is it pretty? Hell no. Is an electrical fire going to break out now? Nope. Still though, might want to tidy it up a bit as a loose shield like that could cause signal issues.