"The Pearl" refers to the Biblical passage Matthew 13.45-46.
Again, the kingdom of heaven is like unto a merchantman, seeking goodly pearls:
who, when he had found one pearl of great price, went and sold all that he had, and bought it.
The pearl of great price is love for God. When a person discovers this love/glory of God, he/she should be willing to sell all the material benefits of his/her mortal life in exchange for God's love.
In each of the first three stanzas, the speaker notes three benefits of the world: knowledge, honor/prestige, and pleasure.
In the stanza on knowledge, the speaker says that he knows the motivations and causes of history. Even with the "keys" to this vast amount of knowledge, he concludes the stanza with, "Yet I love thee." This indicates that all of this knowledge, power as knowledge, does not compare to his relationship with God.
In the stanza on honor, the speaker knows the honor one receives in succeeding in competition and the glory he feels in being successful. Again, he ends the stanza noting this is secondary to God.
In the stanza on pleasure, he acknowledges the pleasure brought to him through all five senses. He concludes with the same deferral to God.
In the last stanza, the speaker acknowledges that he realizes the wonder of all these earthly delights (knowledge, honor, and pleasure) and therefore realizes that it must be something great to trade all of these for God's love:
I fly to thee, and fully understandBoth the main sale and the commodities;And at what rate and price I have thy love,