The basic idea here is that farmers in the US produced more than they could sell.  This, along with foreign competition, depressed prices for crops.  As prices went down, farmers made less money and had to go deeper and deeper in debt.  Eventually, many stopped being able to repay their loans and banks that lent to farmers started to fail.  Because a much higher percentage of people were farmers in those days, the failure of the farm economy was a major blow to the US economy as a whole.  This helped weaken the economy so that the crash of 1929 became a full-fledged depression.