Diagnosis
That's the pump oil drain plug. If it's leaking, the seal (usually an O-ring or crush washer) has failed. The pump uses a special non-detergent SAE 30 oil, not engine oil, and if it all leaks out, the internal components will overheat and seize.
How to Fix It
You need to replace the seal. First, catch the old oil in a pan by removing the drain plug and the oil fill plug (on the top of the pump). Once drained, inspect the drain plug. It likely has a small rubber O-ring or a fiber washer. Replace it with an identical new one—hardware stores often have assortments. Clean the threads, install the new seal, refill with the correct pump oil to the level indicated in the sight glass on the side, and reinstall the fill plug tightly.
If That Didn't Work
If it still leaks from the same spot after replacing the plug seal, the threads on the pump housing itself might be stripped. In that case, you can try wrapping the plug threads with Teflon tape designed for oil, or you may need a slightly oversized plug or a helical thread insert.
Heads Up
Check the pump oil level via that sight glass every few uses. It should be in the center of the glass. Cloudy or milky oil means water has gotten into the pump oil chamber, which is a more serious issue requiring a full tear-down and seal replacement.