Final answer:
Polonius, in his lines, allegorically discusses how people often use the guise of piety to conceal their malicious intents, implying that outward shows of virtue can be misleading.
Step-by-step explanation:
The passage in question features Polonius speaking metaphorically about the tendency of individuals to disguise their true intentions with a show of piety or righteousness. When Polonius says, "We are oft to blame in this - 'Tis too much proved, --hat with devotion's visage and pious action we do sugar o'er/ The devil himself," he is indicating that it is a common fault for people to use the appearance of devotion and pious actions to cover up their more sinister intentions or nature.
This concept suggests that even the worst of us can seem virtuous and good on the surface if we present ourselves in a certain way. These lines encourage the listener to be wary of those who may appear devout or pious, for their true intentions might be quite different.