What is a sum of monomials

Basically a polynomial. A monomial can also be known as a term, so a sum of it is adding terms together.

Example:
5xy^2 — Monomial
5xy^2 - 3x + 5y^3 - 2 — Sum of monomials, aka polynomial

Timbre  (/ˈtæmbər/  tam-bər, also known as  tone color  or  tone quality  from  psychoacoustics) is the perceived sound quality of a  musical note, sound or tone. timbre distinguishes different types of sound production, such as  choir voices  and  musical instruments, such as  string instruments,  wind instruments, and  percussion  instruments. it also enables listeners to distinguish different instruments in the same category (e.g. an  oboe  and a  clarinet).the physical characteristics of sound that determine the perception of timbre include  spectrum  and  envelope. singers and instrumental musicians can change the timbre of the music they are singing/playing by using different singing or playing techniques. for example, a violinist can use different bowing styles or play on different parts of the string to obtain different timbres (e.g., playing  sul tasto  produces a light, airy timbre, whereas playing  sul ponticelloproduces a harsh, even and aggressive tone). on  electric guitar  and  electric piano, performers can change the timbre using  effects units  and  graphic equalizers.in simple terms, timbre is what makes a particular musical sound have a different sound from another, even when they have the same  pitch  and  loudness. for instance, it is the difference in sound between a guitar and a piano playing the same note at the same volume. both instruments can sound equally tuned in relation to each other as they play the same note, and while playing at the same amplitude level each instrument will still sound distinctively with its own unique tone color. experienced  musicians  are able to distinguish between different instruments of the same type based on their varied timbres, even if those instruments are playing notes at the same  pitch  and  loudness.