The file mylatex.ltx permits you to create a format that pre-loads a set of package files (and/or other macros) that you regularly use. In some circumstances, this can be a great advantage (though on an ordinarily fast modern computer on the desktop, gains will be limited).

The general scheme is to initialise your usage by a command of the form:
latex -ini mylatex.ltx <document>
whick will create a format file mylatex.fmt, which you then use as:
latex -fmt=mylatex <document>

The author is David Carlisle.