A supply of beans is when you take a bunch of beans, mash them up into a supply, and then ferment them all together to make them into natto (also known as fermented soybeans).

The reading is an exceptional one. 豆 is okay, and is the on'yomi reading from the kanji. The 納 part is weird though. Instead of being のう it's なっ. But, if you can remember the meaning of this word, you can just think about how you do not (なっとう) want to eat natto. At least a lot of people don't want to eat it. It's quite the acquired taste.