An in-text citation (aka. parenthetical reference) is used when you use someone else's words or ideas within your work. This helps your instructor differentiate between your writing, and the work of others. Doing this helps you avoid plagiarism (stealing someone else's work or ideas). It also shows your instructor that you can integrate the work of others within your own (to support your ideas). Using APA style, it looks like this:
Jonason et al. (2011) discovered that ...
Recent studies on the complex nature of human sexuality found that ... (Jonason et al., 2011).
If, as Jonason et al. (2011) found, "emotional acts can can also demonstrate how the booty-call relationship is distinct from serious relationships (p. 492)," what implications are there for how people demonstrate emotional acts to their booty-call partners through mobile phone use?
As you can see, you can create in-text citations in many different ways -- what is important is that you clearly show which words/ideas are YOURS and which words/ideas come from your SOURCES.
When your instructor sees your in-text citation, they understand you are using someone else's words or ideas, and if they have questions, they can follow-up on the source by looking to the end of your paper where your References page is located. The information in the in-text citation matches the information in the citation on your References page, which for the example above would look like this:
Jonason, P. K., Li, N. P., & Richardson, J. (2011). Positioning the booty-call relationship on the spectrum of relationships: Sexual but more emotional than one-night stands. Journal of Sex Research, 48(5), 486–495. https://doi.org/10.1080/00224499.2010.497984