Sociolinguistics is the study of language's relation to or influence from society. Sociolinguists look at language on both a micro and macro scale.
Micro Sociolinguistics would study social influence on particular linguistic features. For instance, how some cultures speak to babies in a higher register, but other use a different register, or don't speak to them at all. Or why people leave of "g" in words such as running (runnin') and in what contexts they'd use this.
Macro Sociolinguistics would study social influence on a broader, more general scale. For instance, how is conversation structured and why, or how do we know when to take the floor in a conversation.
By studying the ways people use language in different social contexts, spheres, and settings, we can not only come to understand how social relationships work, but also how language itself works.