Hertz measures cycles per second - one Hertz is one cycle per second. Unlike gauss that is a measurement unit for the strenght of a specific thing (an electromagnetic field), hertz is a generic frequency measurement.
Prepend is a word that means to attach content as a prefix. It is often used in different kinds of programming and in automated processes. Some prepending is done manually as a user. For example, a prepend command could be used in a scripting language that a programmer would enter into a certain function or code module. It would add certain characters of text to the beginning of some variable or object.
addendum - textual matter that is added onto a publication; usually at the end. supplement, postscript. matter - written works (especially in books or magazines); he always took some reading matter with him on the plane. back matter, end matter - written matter following the main text of a book.