The information in the sources you use contains a number of recognizable parts such as title, author or creator, journal or website name, date, etc. When you want someone else to understand the information source you used, how will you tell them? What information will you include and in what order? This is what a "citation style" is. Each style directs you as to which pieces of information about your source to include, in what order, and with what punctuation.
What are the pieces of a citation?