A style guide is a set of standards for the writing and formatting of documents. Such guides may be for general use (e.g., Chicago Manual of Style) or for a specific publication (e.g., American Medical Association Style Guide) or for a specific field (e.g., Citing Medicine: The NLM Style Guide for Authors, Editors, and Publishers). Using guides assures uniformity within a document and across multiple documents (e.g., multiple articles in the same journal). Most often students refer to style guides for the accepted format for citing the sources or references used in writing a paper, but they also include other kinds of information such as the particular rules for using commas or quotations or what to include in a photo caption.
Most scholarly journals have requirements about what style authors must use when submitting a manuscript. You should visit the relevant journal’s website for information.