Although marginalia could refer to any writings or sketches in the blank spaces of a printed book, it is most often used to describe instances where someone with a close connection to a book has made marks in it.

Sometimes this is a reader who is interacting with the text.  Famous examples include marks to indicate a key passage or cases where the reader, who is also well informed on the topic, disagrees with what the author has written.

Another form of marginalia can occur when the author of the work marks his or her copy to keep track of corrections or improvements that might be used in a future edition of the work.

In the example below, author-artist-translator Ron Miller was working on his own translation of Twenty Thousand Leagues Under the Seas by Jules Verne for an illustrated version to be published by Unicorn Press.  He started with a copy containing a bad Victorian-era translation and started to mark his alternate phrasing and inserts.

20k Leagues Under the Sea marginalia Ron Miller
Two pages from a copy of Twenty Thousand Leagues Under the Seas owned by Ron Miller and marked up to let him create his own translation for the 1988 Unicorn Press edition that he illustrated.

As with an association copy, the key is who owned the item and made the marks.  If it was any random child, it might not be significant.  However, if the person making the marks had an above average connection with the work, a greater understanding might be gained by looking over those notes. Instead of a flawed copy, in the right circumstances it can be enhanced both in terms of interest and value.

See also signed.