Full Idea
It seems that Locke employs the concept of a real essence when he is addressing issues of scientific explanation, and he appeals to substratum when he is discussing the general concept of what it is to be a thing (as opposed to a property or mode).
Gist of Idea
Locke seems to use real essence for scientific explanation, and substratum for the being of a thing
Source
report of John Locke (Essay Conc Human Understanding (2nd Ed) [1694]) by Jan-Erik Jones - Real Essence §4.4
Book Reference
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.18
A Reaction
[This idea is attributed to Nicholas Jolley 1999] Locke was, of course, utterly pessimistic about the possibility of knowing real essences. For Aristotle, real essence does both jobs.