Azman and I were looking at a trail of muddy footprints leading into the woods.

trail 是名词，意思是"痕迹、踪迹"，指人或动物移动时留下的可见痕迹，比如脚印、车辙等。在这个句子中，trail 特指泥地上留下的一串泥脚印形成的痕迹。