A liar is someone who doesn't tell the truth. A liar tells lies.
"Liar, liar, pants on fire," a phrase of unknown origin, is a children's jump-rope rhyme also used as a playground taunt. Adults, and especially political commentators, have also been known to use the phrase or part of it as a particularly demeaning insult aimed at politicians who make outrageous claims that can't possibly be true. Notice that liar ends in -ar, not -er, as you might expect.