about logical markup (the logical identification of text elements) in LaTeX.
Markup instructions are commands that are associated with certain structural elements of a document. They form an integral part of the LaTeX language. A large selection is predefined in the kernel and the main packages, but users may also define custom markup commands to be able to identify certain elements of their documents logically.
The prototype markup instruction is \emph to emphasize a portion of text - oftentimes confused with the \textit macro, which is not a markup instruction. Canonical examples are the \section macro, the proof environment or \tableofcontents.
A staring point to the subject can be found here: Separate content from formatting - i.e., "just type".