A lord is powerful person who's in charge, or is a ruler or master. Sometimes tigers are described as the lords of the jungle, because they're the strongest, most ferocious predators that live there.
You'll most often see lord describing a man who holds authority or strong influence in a particular area, like a wealthy CEO who's known as a "lord of capitalism," or the head of a large drug-dealing enterprise who's called a "drug lord." Originally, a lord was a landowner during the feudal period in medieval Europe. Even today, a British lord holds an elite title marking him as a baron, earl, or duke.