The Neutrality Act of 1935 was the first one of a series of Neutrality Acts passed in the 1930s. The purpose of these laws was to prevent the United States from getting involved in another war in the way that it got pulled into World War I.
Many Americans felt that the country had been pulled into WWI by trade between the US and the Allies. In order to prevent the US from being pulled into another war, they wanted to outlaw trade between the US and countries that were at war. Therefore, the 1935 Neutrality Act banned the sale of any weapons or ammunition to any country that was at war.
By doing this, the US hoped to be able to stay out of other countries' conflicts.