What does backslash at sign mean in latex? \@

Question

I'm writing a document and my latex code linter is telling me

missing `\@' before `.' in "ABC."

The linter only gives me this warning when there is a capital letter just before a period ending a sentence that precedes another sentence. What does this \@ do? Putting it in my code doesn't seem to make any change to the document.

Minimal example

\documentclass{article}
\begin{document}
ABC. Hello.
\end{document}