The comma is a pesky little piece of punctuation that tends to trip up many an innocent bystander. One such time is in the use of bracketed commas.
You can use commas to mark both ends of a clause that causes an interruption in a sentence or provides additional information. This is called bracketing commas. The commas mark an area where a reader could easily pull out that clause and still have it be grammatically correct, though potentially less interesting. For example, take the sentence, "John Keats, who never did any harm to anyone, is often invoked by grammarians." Removing the clause "who never did any harm to anyone" does not make the sentence grammatically incorrect, just less interesting.
Sometimes bracketed commas may seem necessary when they really aren't. If the clause is integral to the meaning of the sentence, you don't need to present it with a pair of commas. For example, take the sentence, "Belinda opened the trapdoor and, after listening for a minute, closed it again." Removing "after listening for a minute" still provides a grammatically correct sentence, but one that is not as informative. That clause is integral to understanding the meaning of the sentence.