The document discusses key concepts in second language acquisition (SLA). It defines SLA as referring to both the study of individuals learning a language after their first, and the process of learning that additional language. SLA can involve learning a second, third, or subsequent language. The document outlines factors that affect SLA, including individual factors like age and cognition, external factors like instruction and culture, and affective factors like anxiety and self-esteem. It also discusses concepts like interlanguage, language transfer, and the role of both innate ability and social experience in language acquisition.