Odysseus had returned to his home in Ithaca but he was disguised as a beggar. Eurycleia, one of Penelope's maids, recognices him when she was bathing him and saw a particular scar he had.
Odysseus had a scar above his knee, caused by a boar while he was hunting with his grandfather. This testimony of the scar is what Eucycleia presents to Penelope to reveal Odysseus presence in Ithaca.