Let hist'ry tell, where rival kings command,
And dubious title shakes the madded land,
When statutes glean the refuse of the sword,
How much more safe the vassal than the lord (29-32)
It's not good to have too much power. It's much safer to be a nobody (a "vassal") than a lord or a king. That's because power corrupts (people gain "dubious" titles or honors that they don't deserve) and this leads to trouble.
Low sculks the hind beneath the reach of pow'r,
And leaves the wealthy traitor in the Tow'r (33-34)
Here, again, the speaker warns us that it's better to have less, rather than more, power and wealth. Power and wealth lead us to the Tower—a reference to the Tower of London (a famous prison) where many important political figures in British history were detained (and a few were even killed).
Yet still one gen'ral cry the skies assails,
And gain and grandeur load the tainted gales;
Few know the toiling statesman's fear or care,
The insidious rival, and the gaping heir. (45-48)
Everybody wants "gain and grandeur"— wealth and power. But people who ask for those things don't realize all the trouble they can cause. A big statesman or politician has to deal with fears and responsibilities. He has rivals who want to take his place. He has to worry about who will be his heir and inherit his position.
In full-blown dignity see Wolsey stand,
Law in his voice, and fortune in his hand:
To him the church, the realm, their pow'rs consign,
Through him the rays of regal bounty shine (99-102)
Here the speaker describes Wolsey at the height of his power as the king's right-hand man. He could dictate laws, he had the powers of the church and the English realm in his hand, and he was the king's spokesman. The speaker presents Wolsey as an example of a man who was very ambitious for power.
What but their wish indulg'd, in courts to shine,
And pow'r too great to keep, or to resign? (133-134)
Sometimes having our wishes comes true brings us trouble rather than peace or joy. The speaker suggests that, throughout history, men's greed for power has led them into trouble.