Tennyson's "Ulysses" is a wildly popular poem, and one which instigates a variety of interpretations. However, as a starting point, it helps to look at one of the poem's most prominent themes: the rebellion against age, infirmity, and mortality.
The poem opens upon an aging Ulysses lamenting his essentially boring and purposeless life at home in Ithaca. Far from being grateful for having returned home from his harrowing journeys, Ulysses laments his idleness, resenting his "aged wife" (3) and the "savage race" (4) he is doomed to wait upon. However, rebelling against this unremarkable existence, Ulysses declares "I cannot rest from travel" (6) and prepares to set off on yet another voyage.
Though "Ulysses" has many famous lines, the key lines for our purposes occur at the end:
We are not now that strength which in old days
Moved earth and heaven, that which we are, we are;
One equal temper of heroic hearts,
Made weak by time and fate, but strong in will
To strive, to seek, to find, and not to yield. (66-70)
In this stirring conclusion, Ulysses essentially reflects on the loss of his legendary strength, cleverness, and heroism; he is no longer the dashing hero, and is instead and old king sick with nostalgia. However, despite this realization, Ulysses still resolves to strike out into the unknown yet again to once again test his courage. As such, the poem is largely a rebellion against old age and slipping quietly off into obscurity, as it centers on a protagonist
determined to defy his mortality and continue "to strive, to seek, to find, and not to yield."