APA (American Psychological Association) citations are typically used in science classes. APA (and other citation styles) enables you to give credit to sources that you use: citations are a big plus in writing a paper, because they help you show that you have read, understood, and incorporated strong sources, written by experts, into your paper. Citing good sources makes your voice as an author more compelling!
At first, try not to worry too much about the many tiny details of APA citations: it takes time to learn what an APA citation looks like and how to write one. With practice, you'll become proficient at it. The important thing is to make sure that you give credit to the sources that you use.
Just to get you started, an APA citation for an article from a scholarly journal will look like this, listing the author(s), date, article title, journal name, volume, issue, and page number, and (sometimes) a link containing the "DOI," which is sort of like a social security number for an article:
McBurney, M. I., Hartunian-Sowa, S., & Matusheski, N. V. (2017). Implications of US nutrition facts label changes on micronutrient density of fortified foods and supplements. Journal of Nutrition, 147(6), 1025-1030. https://doi.org/10.3945/jn.117.247585
For more examples of APA citations, see the library's APA Citation Examples guide.