.ltx files are used by the TeX typesetting system, most commonly with the LaTeX macro package.
There files usually contain pre-coded commands to be included in actual documents (most commonly .tex files). They're used to keep commands and definitions separate from the actual document content.
However, these files can contain any TeX/LaTeX code and be used for anything that .tex files are used -- using .ltx for "include files" is merely a convention.
How to open .ltx files?
Software to open .ltx files
What are .ltx files