Sociolinguistics is a hybrid of sociology and linguistics. It is the study of the relationship between language and society. The language we use is influenced by social contexts. This is true, for example, in the use of accents. Sociolinguistics is a very broad field.
According to Scott Thornbury, sociolinguistics was important to the rise of the communicative approach and the functional syllabus. The communicative approach to teaching languages became popular in the late twentieth century. A functional syllabus includes the appropriate language for: greeting people, asking for information, making requests, etc. There are, for instance, many ways to make a request ("Can you . . .?," "Would you mind . . .?," etc.), and the correct way depends on the situation, age, and relationship of the speakers. Japanese, for example, has a complicated system of language forms used based on these factors.
Another aspect of sociolinguistics is bilingualism. Children use their first and second language in bilingual schools. Over 70% of the world's population is bilingual or multilingual.