@JMS's answer is excellent for commodities such as food. I'd like to add a few points that caused civilian hardship even without blockade.
World War I saw for the first time millions of men and women mobilized and sent to war. This means millions of people no longer doing their regular jobs. Instead they now need to be fed, clothed, and supplied by the shrunken workforce. Worse, as soldiers they need more than as civilians, and battles consume men and material at an alarming rate.
To avoid losing too many specialized workers, many people who have critical jobs are exempted from serving: farmers, factory workers, etc. In addition the loss of workers compensated for by transferring more people to manufacturing, and also opening up manufacturing to new groups such as women. It's no coincidence that Women's suffrage got a big boost after World War I.
Nitrogen is critical for two things: explosives and fertilizer. The Great War required a lot of explosives, and unless the production of nitrogen picked up the availability of fertilizer would suffer.
Even though the atmosphere is mostly nitrogen, for a long time we didn't know how to use it. Instead we mined it. The Allies had access to natural sodium nitrate and bird poop, but the Germans were cut off.
Instead they turned to the then-new Haber process to fix nitrogen from the air and produce ammonia which could then be turned into fertilizer and explosives. But this was a brand new process and production was still ramping up when the war started.
Critical war materials
There are some materials which are simply not locally available and there are no replacements.
Petroleum, while not nearly as important in WW1 as in WW2, was still necessary for lubricants and fuel hungry naval vessels. Germany had some supply from Romania until they joined the Allies in Aug 1916.
Rubber, again not as important in WW1 as in WW2, was also necessary for gas masks, elastic, and seals. At the time nearly all rubber was imported natural rubber from trees in South America, South and Southeast Asia. Synthetic rubber wouldn't become economically viable until WW2.
Others are not so obvious. WW1 required a lot of steel. Steel requires a lot of coal and iron, of which Germany had plenty. But good alloy steel requires alloying elements like manganese, nickel, chromium, molybdenum, vanadium, silicon, boron, cobalt, copper, cerium, niobium, titanium, tungsten, tin, zinc, lead, and zirconium. A shortage in any one of these materials can cause good steel to become scarce.
Others are necessary to keep people happy: coffee and tobacco, for example.
Horses were also needed by the military in great numbers, plus the specialists to handle them, shelter, and equipment. Horses were not just for cavalry, but were much more critical for transportation: trucks being rare and roads being poor. And they died in alarming numbers. This took them away from civilian use, much of the economy was still horse-drawn in World War I. In particular farm production was greatly impacted.
And these great numbers of horses needed to be cared for and fed. Horse fodder became a critical commodity.
The Great War has a good piece on Animals of World War 1.
In WWI if you wanted to move and supply an army of millions there were two ways to do it: horses or trains. We talked about horses already. Moving and supplying armies requires a lot of rolling stock: locomotives, cargo cars, passenger cars, etc.
Trains are also how you move food, goods, and civilians around. A country has a limited amount of rolling stock. The more the military uses, the less the civilian economy has available. This hurts production.
Germany had no plan
Germany, in particular, produced most of its own food and thought its continental supply lines would keep trade flowing. But they didn't have a wartime production plan. Germany didn't plan on a war, it was a local incident that spiraled out of control. And they didn't plan on a long war, they thought they could knock France out in a few weeks and turn all their attention on the Russians. And they did not plan on Britain and the Royal Navy getting involved, the Germans did not believe Britain would actually go to war to protect Belgium.
Without a plan, food production was hit hard by the loss of men and horses and fertilizer. They didn't expect to have to keep an army of millions in the field for years. Nobody did.