What is the poem "To Penhurst" about?

"To Penshurst" is a country house poem, a genre that emerged in the seventeenth century to praise country estates. This poem describes the natural beauty and bounty, the hospitality  and the moral order of Penshurst, the home of the Sidneys, a literary and aristocratic family. 

In the poem, nature, which reflects God's order, is preferred to the artifice of a grand house. The Sidney house is not "built to envious show," nor does it boast "marble," "polished pillars," or a "roof of gold." Instead, Jonson celebrates the estate for its "better marks" or higher attributes:

Thou joy’st in better marks, of soil, of air, 

Of wood, of water; therein thou art fair. 

Nature here is rich, beautiful and bountiful, and as critic Raymond Williams describes in The Country and the City, "To Penshurst" reflects a ruling class ideology that obscures the real labor of the rural life, replacing it with a fantasy that the work is easy because fish jump out of the water in their eagerness to be caught and "every child" can reach the fruit to pick it:

Fat aged carps that run into thy net, 

And pikes, now weary their own kind to eat, 

As loath the second draught or cast to stay, 

Officiously at first themselves betray; 

Bright eels that emulate them, and leap on land 

Before the fisher, or into his hand. 

Then hath thy orchard fruit, thy garden flowers, 

Fresh as the air, and new as are the hours. 

The early cherry, with the later plum, 

Fig, grape, and quince, each in his time doth come; 

The blushing apricot and woolly peach 

Hang on thy walls, that every child may reach. 

In this paradise, hospitality overflows. Jonson praises the liberality of the Sidneys, and contrasts their generosity to his experiences at other houses:

the same beer and bread, and selfsame wine, 

This is his lordship’s shall be also mine, 

And I not fain to sit (as some this day 

At great men’s tables), and yet dine away. 

In other words, the poet is invited to eat, not just sit at the Sidney table. In other houses, a guest might be given inferior food to the lord's or sent to "dine away," meaning to find dinner at an inn. At Penhurst, everyone is welcomed and treated generously. 

The poet moves to dwell on the moral order reflected in the family. Jonson writes of Robert Sidney

Thy lady’s noble, fruitful, chaste withal. 

His children thy great lord may call his own, 

A fortune in this age but rarely known. 

The family, we learns, prays and the children are taught to be gentle. The poem ends by praising this moral order, reflected in the natural order of Penshurst, as more important than "proud, ambitious" homes that others might occupy.