If you wanted to rob Fort Knox and take all of its gold, you'd plan a heist to get the bullion. Bullion is gold or silver in bulk, often in the shape of a bar.
Once upon a time, coins were made of actual precious metals. Before the coins were made, the gold and silver for them was stored bulk, in bullion. Try not to confuse this word with, bouillon, which is broth, or a cube of beef or chicken flavoring used to make broth. If you see the OU in the first syllable, it's a clue that it's the sOUp. If you just see a "bull," think of the stock market and glittery gold.