That's a source for manifold vacuum....
If you don't need to use it, just cap it off... Or you can connect a vacuum gauge to it and it's good to use to see engine vacuum and if you drive for highest vacuum, you will get maximum MPG...
Now the other nipple on the top left of the picture is a source for ported vacuum and should be connected to the vacuum advance pod from the distributor... Ported vacuum is what the distributor vacuum advance needs to work properly...
