The term kibbitzer comes from the chess cafes of central Europe at the start of the 20th century. A kibbitzer did not play chess, but watched other people playing, and possibly made comments on their play.
In the following citation from The Guardian it comes to have the general meaning of an interested observer:
This week, at last, all is official. Scientists, journalists and interested kibbitzers gathered at Fermilab on Tuesday to hear the news.
In language education, Kibbitzers are web pages that allow you to observe the sorts of problems that arise in revision of students' written work. And observe the process of investigating attested language so as to arrive at the contexts in which a word or phrase is typically used. Note, we did not use the word 'correctly' used. Contexts here refers to semantic, syntactic and discoursal contexts.
Adapted from here.