After the fall, Adam was cursed by making his work more difficult. Adam was in charge of growing crops for food, and as implied by the text, prior to the Fall it was not a difficult task. The curse resulted in the work being more difficult because thorns and thistles started to grow. You can imagine Adam sticking his hand down and getting stuck by a painful thorn. He also had to work by the sweat of his brow, implying that the heat would make it more difficult for him, such as causing him to become fatigued and have to rest, thus slowing down his progress.