At the beginning of WWII, the United States was officially neutral. This was due in part to the fact that Congress had passed laws prohibiting the US from trading with or loaning money to countries who were at war. As WWII went along, these laws prevented the US from becoming directly involved and forced FDR to find creative ways to get involved in the war.
In 1940 and 1941, FDR found ways to help the Allies without technically violating the neutrality laws. An example of this was the "destroyers for bases" deal that the US made with Great Britain. In this deal, the US gave old destroyers to Britain (not a violation of the laws) and in return got the rights to British bases in the Caribbean. This did not violate the laws, but it helped the British because it freed their forces from having to defend those bases. Those forces could then go fight elsewhere.
Overall, then, the neutrality laws affected US policy by preventing the US from direct and overt involvement in international situations. However, they did not prevent the US from finding creative ways to help the Allies.
During the 1930s, Americans were wary of getting involved in European conflicts, and there was a wave of isolationism. Many Americans believed that World War I had been fought to benefit bankers and munitions manufacturers. In this climate, Congress passed a number of neutrality acts. The First Neutrality Act, passed in 1935, disallowed American manufacturers from selling armaments or weapons of any kind to belligerent nations. The Neutrality Act of 1937 forbade American ships from carrying arms; however, at the President's discretion, some belligerent nations could buy goods, not arms, from the U.S. and transport them home in their own ships. This was referred to as the "cash and carry" law. After Germany invaded Czechoslovakia, the Neutrality Act of 1939 allowed the U.S. to sell arms to other nations, as long as the weapons were not carried on American ships, but the U.S. could not loan other nations money.
President Franklin Roosevelt finally found a way around the neutrality laws to help Great Britain. During the so-called "Lend-Lease Act" of 1940, Roosevelt gave Britain destroyers in return for leases to territories in Newfoundland and the Caribbean. This allowed the U.S. to provide aid to Great Britain, then struggling under Nazi bombardment.
The neutrality acts allowed the U.S. to provide limited aid to nations facing the Nazi juggernaut. Arguably, these laws did not provide enough aid to Spanish Loyalists, who lost their fight to fascism during the Spanish Civil War of 1936-1939. While the U.S. did eventually provide food to the Loyalists, the neutrality laws forbade American manufacturers from providing arms, while the fascists in Spain were being supplied by Germany. The lack of American armaments hurt the Loyalists, and it might be argued that American isolationism was in part responsible for the fascist victory in Spain.