Substantive due process is distinguished from procedural due process (due process = from both the 4th and 15th Amendments).
Procedural due process = what process is the government using for taking your life, liberty or property (and are they giving you enough process)?
Substantive due process = what are the grounds on which the government is taking away your life, liberty or property (and are they permissible grounds)?
In order to determine whether or not they are permissible grounds, you have to look at the right that is implicated (fundamental rights get strict scrutiny, some things like legitimacy and gender get some kind of intermediate scrutiny, and other stuff gets rational basis scrutiny).
More or less. Obviously there are more nuances.