400 Oft from those grave affairs were wont abstain,

wont > accustomed to, given to

401 With the sweet lady Muses to play: 402 So Ennius the elder African,

Ennius > (Quintus Ennius, 239-170 BC, Roman poet)
the elder African > (Scipio Africanus, 237-183 BC, Roman general in
Second Punic War)

403 So Maro oft did Caesar's cares allay.

Maro > (Publius Vergilius Maro [Virgil], 70-19 BC, author of the
Aeneid)
Caesar > (Augustus Caesar, 63 BC-14 AD, Emperor of Rome)

404 So you, great lord, that with your counsel sway

sway > influence

405 The burden of this kingdom mightily,

burden > fate, destiny