BibTeX is an external programme that belongs to LaTeX and automatically generates bibliographies.
The sources (e.g. books, articles, websites) are stored in a .bib file, and BibTeX uses this to format the bibliography and citations in the text during compilation, depending on the desired citation style (APA, IEEE, Chicago, etc.).
Use of BibTeX
For the following commands, the engine must be biber.
% In the preamble
\usepackage[backend=biber,style=apa]{biblatex} % APA citation style
\addbibresource{literatur.bib} % bib file with sources
% In the text
Lorem ipsum \parencite{Source1}. % Source1 from the bib file
\printbibliography % List of sources (where desired)