A word or phrase naming an attribute, added or grammatically related to a noun to modify or describe it.
Hope that helps :)
Argos was Odysseus's hunting dog and his faithful pet. Even when its owner left it to fight in the battle of Troy, it waited for him for years and years, with unwavering loyalty. When Odysseus finally returned to Ithaca, disguised as a beggar, a very aged Argos recognized him and felt incredibly happy to see him again, dying not long after, as it could finally fulfill its wish of seeing him once again. Argos is included in the poem to represent undying loyalty.
Yes, all the "a"s, "an"s, and "the"s in those sentences are articles.