What happens in the card game in lines 29-54?

Keep in mind that "The Rape of the Lock" is a mock epic. Pope is satirizing--skewering, really--the silliness of high society of his day by placing an incident within the form of an epic, complete with the invocation of the muse, the intercession of supernatural creatures, and the "arming" of the characters. Of course, the supernatural creatures are not the Zeus and Apollo of Homeric fame; they are nymphs. The main character, Belinda, "arms" herself with makeup and fancy clothing and jewelry. Think of this poem as an extravagant, 18th century way of saying, "First World Problems." 

In the first through third stanzas, Belinda heroically sleeps in until noon while her guardian sylph--a spirit of the air--tries to warn her in a dream that someone in her social circle is up to mischief. Part of that warning is to beware of the card game discussed in stanza 3: 

Fairest of Mortals, thou distinguish'd Care
Of thousand bright Inhabitants of Air!

He calls her the "Fairest of Mortals" and says she has the protection of a thousand sylphs and nymphs of the air. 

If e'er one Vision touch'd thy infant Thought,
Of all the Nurse and all the Priest have taught,
Of airy Elves by Moonlight Shadows seen,
The silver Token, and the circled Green,
Or Virgins visited by Angel-Pow'rs,
With Golden Crowns and Wreaths of heav'nly Flowers,
Hear and believe! thy own Importance know,
Nor bound thy narrow Views to Things below.

He says she is naive, having "infant thought," having been tutored only by her nurse and priest, both too chaste to know what will happen or warn her. She should beware of "the silver token, and the circled green." In a true heroic epic, this would be a field of battle and a prize to be won by knights or warriors; in this case, it is the card table she will sit at later that day. 

Some secret Truths from Learned Pride conceal'd,
To Maids alone and Children are reveal'd:
What tho' no Credit doubting Wits may give?
The Fair and Innocent shall still believe.

He says that the proud and learned wouldn't believe me, but you are still innocent enough to do so. 

Know then, unnumbered Spirits round thee fly,
The light Militia of the lower Sky;
These, tho' unseen, are ever on the Wing,
Hang o'er the Box, and hover round the Ring.
Think what an Equipage thou hast in Air,
And view with scorn Two Pages and a Chair.

So listen: You have countless invisible spirits on your side, but be careful of the three-handed card game "Two Pages and a Chair."

As now your own, our Beings were of old,
And once inclos'd in Woman's beauteous Mold;
Thence, by a soft Transition, we repair
From earthly Vehicles to these of Air.

We were once young, beautiful women like you. We're now airy spirits whose job it is to protect you. 

Think not, when Woman's transient Breath is fled,
That all her Vanities at once are dead:
Succeeding Vanities she still regards,
And tho' she plays no more, o'erlooks the Cards.

Just because we are no longer women, we understand vanity and we protect it in living women. We may not play cards anymore, but we're still there to help you out. 

(The actual card game doesn't occur until Canto 3, incidentally. The lines you asked about are only the sylph's warning.)