It means..
If you company already has an approved labor certification for a particular job category, for a specific skill set & if your company did not use it for the employee for which it was originally filed, which may happen for instance if the employee has left, then that labor certification may be utilized by substituting you in place of the earlier employee provided you meet all the requirements of the job, the skills required, education level etc etc.
Above is the general framework for labor substitution. Your lawyer should be able to guide you through the specificities in detail. If you can be substituted for any approved labor certification then the upside is that you would not have to waste time in getting labor clearance.
Good Luck/titu