In "Ulysses," Tennyson imagines the hero of Homer's epic, The Odyssey, years after the epic ends and Ulysses has been faced with years of remaining in Ithaca as king.  Ulysses sees his current position as static, whereas during his adventures he was dynamic.  These are not Tennyson's words, but they apply to what he writes.  This is at the heart of the poem.

Ullyses sees himself now as an "idle king," he narrates how ""dull it is to pause, to make an end,/ to rest unburnished, not to shine is use!"  He is a "gray spirit yearning in desire/To follow knowledge like a sinking star,..."  His current role is static, unchanging, unadventurous, boring.

In contrast, in the old days he was:

...always roaming with a hungry heart
Much have I seen and known--cities of men
And manners, climates, councils, governments,
Myself not least, but honored of them all--
And drunk delight of battle with my peers,..."  (12-17)

The poem contrasts the static life Ulysses now lives, according to Tennyson's imaginative poem, with the dynamic life he lived during his travels in The Odyssey.