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.
Pre-exposure prophylaxis (PrEP) is a course of HIV drugs taken daily by HIV negative people most at risk of HIV to reduce their risk of HIV infection. Truvada is currently the only drug approved for use as PrEP. Truvada is a single pill that is a combination of two anti-HIV drugs, tenofovir and emtricitabine.