The document discusses phonology, the study of sound patterns in language. It explains that phonology looks at what sounds are in a language, how they combine into words, and why certain sound features are important for identifying words. It then provides examples of how English plural, possessive, and verb morphology can have different phonetic forms (allomorphs) depending on the surrounding sounds, with rules to describe the environments that trigger each allomorph. The document also introduces the concept of phonemes as the basic units of sound in a language, which have multiple phonetic manifestations (allophones) depending on context.