Can someone identify this for me?

[bschevyk10]

When I swapped out my motor I remember this being hooked up to a wire and I just seen that it is disconnected. I must have forgot to hook it up. And I had it at the shop to tune/time everything and they didn't hook it up either. Or either it came disconnected somehow. Anyways just trying to figure out what it is and if it's important to have hooked up. In the picture it goes into the top of the block right behind the intake manifold and next to the distributor. It looks yellow and dirty with one connection point on top. Thanks for your help.