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PPM stands for parts per million, a way of expressing a small ratio or fraction, much like a percent expresses a fraction as "per 100" and baseball batting averages are effectively expressed as "per 1000". Any portion of astronomy or astrophysics may use the term, typically for a fraction with an order-of-magnitude in the range of a few millionths (The term basically means the same thing as millionths). Some common uses:
PPM is common within and outside of astronomy to indicate a fraction of some material that is a particular constituent. It refers to the fraction of the number of particles, such as the fraction of molecules within a gas of a particular compound. Atmospheric pollutants on Earth are often cited that way, and as well as rarer constituents of bodies such as clouds.
CCDs produce their data as counts per pixel, and PPM is often used to express the noise in the counts and also to express the depth of transients, i.e., changes in counts produced by a change in the EMR from the source. Having the "noise" and the "data" expressed compatibly, the two can be compared when deciding whether an apparent transient is real.