Like me, you probably already knew that Passover was a Jewish holiday in April, celebrating the ancient Israelites' freedom from slavery in Egypt. According to the Book of Exodus, they had to leave in such a hurry that they didn't have time to let their bread rise, or leaven. That's why Jews today only eat matzo -- a flat, unleavened bread -- during Passover. The Torah also proscribes that no leavened products, known as chametz, be present. What constitutes chametz can differ depending on who you ask, but strictly speaking, it means any product containing grains that isn't certified as kosher for Passover.
No chametz means not so much as a grain of flour. Jews may spend weeks before Passover cleaning their homes to remove it. First, they'll go through their kitchen, removing any chametz and packing it away. Then they'll clean the entire house -- between every crack and crevice -- to be sure that even the tiniest speck is gone. There's a ritual search for any remaining chametz the night before Passover, and special prayers to release ownership of any chametz that was missed.
What happens to the chametz? Some donate it or sell it to non-Jews (who also lease a sealed-off space in the home where it's stored and buy it back after Passover, depending on what it is). Many Jews burn their chametz in a bonfire. It's a seriously specialized form of cleaning that's a tribute to the past but is also designed to remove the spiritual chametz of egotism and oppression.