Noise has a rather complex meaning in the context of sound and music. While it has specific scientific definitions (which we'll get into later), it carries a plethora of––mostly negative–– connotations. Think about when you've heard the term "noise" used or how you might use it in the context of sound and music. You might have been listening to loud music and had a cranky parent yell out "turn that noise down!" You might describe an irritating group of children as "noisy". Or maybe you've heard the term "noise pollution" in loud cities. It may be the case that we frequently think of noise as unpleasant and unwanted––something we want to reduce or extinguish.
In the context of computer music however, noise is a more specific and useful tool. Noise is conceptually a kind of randomness. There are different "colors" of noise that reflect different distributions of randomness. "White noise" is essentially all frequencies randomly distributed across a spectrum with equal power. This is probably a familiar sound to you, perhaps you've heard it coming out of an old TV with no reception, that loud hiss or static sound. There is also "Pink noise" and "Brown noise" and others (we're mostly concerned with white and pink in signal processing).
Familiarize yourself with the different colors of noise in this Wikipedia article:
You can just focus on the sections on white, pink, and brown noise, as those are the most relevant to what we'll be covering.
Additionally, read the following article and test out the accompanying Max patch that demonstrates some different concepts of randomness and noise:
Joji Yuasa - Projection Esemplastic For White Noise