What does top-tier mean when referring to journals/conferences?

Top tier can mean a lot of different things to different people - and indeed, can mean a lot of things to different fields. Ask a diverse group of academics drawing from math, health science, and the humanities what a "good" impact factor is sometime if you have the chance - the variability is massive.

Personally, for me, when I say "Top Tier" I'm usually thinking of soft factors - journals where solid publications go, venues that you'd highlight on the cover letter of a CV, etc. That often has an association with Impact Factor, but it's not necessarily fixed. For example, PLoS One has an impact factor that's about on par with several journals I'd consider "Top Tier" in my field, but PLoS One isn't on that list.

I many parts of Asia "Top Tier" tends mean journals and conferences that are high ranked by their National Research Foundation. The NRF tend to rank journals based on journal index, and other criteria.

In Europe top tier tended to mean the journals that the respective community believed to be of high quality.

Very generally, journals could be ranked by impact, but even high-impact journals may not be considered top tier by your research community / university / funding agency.