Thy genius calls thee not to purchase fame In keen iambics, but mild anagram: Leave writing plays, and choose for thy command Some peaceful province in acrostic land. There thou may'st wings display and altars raise, And torture one poor word ten thousand ways.
The term "iambics" refers here to satires in the classical tradition, commonly written in iambs—much like Dryden's own poem.
An "anagram" is a rearranging of letters in a word, and refers here to a common practice in seventeenth-century poetry where writers would arrange their poems on the page in certain shapes, like wings or altars.
This style was viewed by purists as "false wit."
Another example of this style is acrostic poetry, in which the first letter of each line spells out a word. Dryden accuses Shadwell here of writing cheaply in this manner.
Or if thou would'st thy diff'rent talents suit, Set thy own songs, and sing them to thy lute. He said, but his last words were scarcely heard, For Bruce and Longvil had a trap prepar'd, And down they sent the yet declaiming bard. Sinking he left his drugget robe behind, Born upwards by a subterranean wind. The mantle fell to the young prophet's part, With double portion of his father's art.
In Ancient Greece, Homer's epic poems would often be sung by traveling performers, generally with accompaniment from a stringed instrument like a lyre. In medieval Europe, bards followed in this tradition, singing epic poetry while playing the lute.
We get a reference to Bruce and Longvil, two characters in Shadwell's The Virtuoso. In the play, they pull a trap door to dismiss the haughty Sir Formal Trifle in the middle of one of his rambling speeches.
Dryden leaves us with a biblical reference, an allusion to the story of Elijah, in which Elisha picks up the elder prophet's mantle after he departs to heaven in a whirlwind. This is the origin of the modern expression "to take up the mantle." And as Elisha carries on in the absence of his mentor, Shadwell is left as the inheritor of his father's crummy drugget ("woolen fabric"), and equally crummy legacy. Of course, though, he has a "double portion of his father's art." That sounds awesome, but then you realize that this means he has an even sharper knack for writing terrible poetry than his predecessor.
And there we have it. "Mac Flecknoe" in all its scathingly hilarious glory. Sorry Shadwell. We can still feel the burn all the way from here.