I don't understand anything involved with economics in my AP Us History class?

In a nutshell, the author seeks to persuade you that the depression starting in 1929 was caused by poor government mismanagement rather than by irresponsible greedy investors. In the first paragraph, the author mentions buying on margin and how it was not done more at the time we started the depression, as compared to some years beforehand. The author suggests the opposite that there was less buying on margin. The point the author wants to make is that we did not see margin buying skyrocket and result in an economic collapse, as he suggests others would want you to believe. What is margin buying? Basically like buying stocks on your credit card. When the price of stocks comes down, you still have to pay. That is not an accurate definition, but serves to make the point that there is risk and uncertainty in buying on margin. The actual process is giving cash to a broker to buy stocks, and then using the value of your stocks to borrow the money to buy more stocks. The broker will only lend you, say, 50% of the value of your original stock to fund your margin account. If the value of that stock goes down, the 50% you can borrow is a smaller number. If the smaller number is less than you actually borrowed, the broker wants you to put in more cash to make it up. This is a "margin call." When people cannot cover their margin calls, on a wide spread basis due to significant other economic problems, the result in an affect on other factors of the economy. Hence, margin purchases are seen by some as risky and irresponsible. However, it is the point of the author that others have blamed margin purchasing for the great depression, that it is not true, that certain government actions caused the great depression.