Allusion is a figure of speech that refers to a well-known person, place, event, literary work, or work of art in order to create an association or enhance meaning. In this case, Tennyson is alluding to the epic figures of Ulysses (from Homer's *The Odyssey*) and Achilles (from Homer's *The Iliad*).
By using these allusions, Tennyson is likely drawing parallels between the journeys and struggles of these ancient heroes and the themes he is exploring in his own work.