Suppose chapters 1 and 2 have 2 sections each and that the book class is in use.
By default, the four sections are labeled 1.1, 1.2, 2.1, and 2.2.
If you want them labeled 1, 2, 1, and 2, i.e., with a reset each time LaTeX executes a \chapter directive, you should run
in the preamble.
Note that this numbering/...