It little profits that an idle king, By this still hearth, among these barren crags, Matched with an aged wife, I mete and dole unequal laws (1-3)
You know what Instant Messenger says if you don't touch it for a while? It says "idle," as in not active or moving. Well Ulysses is idle too, and because he's "idle" or just sitting around, he's bored. And he's bored because Ithaca is boring; the hearth is "still" and the crags are "barren," which means they're practically lifeless. Why would Ulysses want to stay here when he could go find a one-eyed monster somewhere?
How dull it is to pause, to make an end, To rust unburnished, not to shine in use (22-3)
Ulysses elaborates more fully on his sense of dissatisfaction; a knife or sword is meant to be used, and if it's not used it gathers "rust," which means it stops being shiny. Ulysses feels that because he's stopped moving he's losing his luster, becoming just another "dull," "unburnished" sword hanging in the garage.
As though to breathe were life! Life piled on life Were all too little (24-5)
Ulysses isn't happy to just be alive; the body's functions do not make life. He sees his subjects as drones who do nothing but "hoard, and sleep, and feed" (5), and the danger is that Ulysses might become more like his subjects than is appropriate for a king. Even if he gets out of Ithaca, though, Ulysses implies that he would still be unsatisfied: even several lifetimes wouldn't be enough for him! He is either exaggerating, displaying a desire for experience that is more insatiable than average, or showing that he can't ever be satisfied, at least not in this life.
And vile it were For some three suns to store and hoard myself, And this grey spirit yearning in desire (28-30)
It would be a tragedy if Ulysses were to sit around for three years and do nothing but store up supplies if he's not going anywhere; he wants to be out there traveling – he is possessed by an insatiable desire, and sitting around at home isn't the way to satisfy it. That would be "vile," disgusting, unpardonable. We are dealing with matters of the spirit, after all, and they shouldn't be taken lightly.