tex-core is for questions that are independent of formats and macro packages. Such questions usually relate to low-level TeX programming or the behaviour of TeX at an underlying level.
Questions tagged tex-core will in general cover topics such as TeX primitives (build-in commands), the basics of macro programming, TeX's expansion mechanism and so so. Good reference works for these low-level TeX concepts include The TeXbook and TeX by Topic (the later is freely available).
Note that even Plain TeX is a format in itself; for questions about it specifically, use plain-tex.