Something leaden moves in a heavy, slow way. After a sleepless night, your leaden feet will walk more sluggishly than usual to the bus stop.
Dull, heavy things can be described as leaden, like a dense, boring novel full of leaden dialog or your fatigued, leaden legs at the end of a full day's hike up a mountain. If something is the color of lead — a dull gray — that's also leaden, like the leaden, stormy sky. Finally, things made of lead are quite literally leaden, the word's original meaning, from a Germanic root meaning "heavy metal."