What is a fricative sound?

Adjective: fricative
Of speech sounds produced by forcing air through a constricted passage (as 'f', 's', 'z', or 'th' in both 'thin' and 'then')

Noun: fricative
A continuant consonant produced by breath moving against a narrowing of the vocal tract

Fricatives are the kinds of sounds usually associated with letters such as f, s; v, z, in which the air passes through a narrow constriction that causes the air to flow turbulently and thus create a noisy sound.