Boop is the effect of Erfworld's built-in censorship, whereby Parson's profanity comes out as "boop" instead. One of the odder in-universe mechanics, this is discovered by Parson almost immediately after being summoned. While the Erfworld natives apparently understand the intentions of the word, Sizemore and Wanda are the only characters to have responded in a manner that implies they understand the specifics of each use. At the end of Book One, Parson manages a curse that is conspicuously not booped out.Erf-b1-p150 This change may illustrate his developing transcendence of the Erfworld ruleset, or at least that he has impressed the Titans enough for them to drop it. This is compounded in the summer text update 009, where Parson manages another curse word in the last line that is not booped out, though attention is not drawn to it.Erf-b1.5-p009
Real World References
The presence of built-in censorship may relate to censorship programs used by some programs that operate via the Internet. In particular, censorship filters are often built into the chat system of multiplayer games. These programs are not 100% perfect and profanity can get through either accidentally or through creative spelling, which may relate to Parson's limited ability to swear without getting Booped Out in the process.
More specifically, Erfworld was originally hosted on the Giant in the Playground website, whose forum software replaces profanity with asterisks. The word "crap" is not affected, just like it isn't in the comic.