It's expected to fix a lot of the latency issues plaguing the 3000 series right now due to their chiplet design.Core 1 sharing resources with core 7 for example currently nearly quadruples latency compared to core 1 sharing with core 2, because they're in different chiplets.

Ryzen 3000 is also a tick, Ryzen 4000 will be the "tock" of Zen2. It's Zen2+ where all refinements are done before new architecture is expected and baby Zen2 is fantastic but it's not without its flaws. The flaws don't really matter for anything but gaming so if you don't care about 1% lows and gaming performance then buying Zen2 right now isn't going to be bad. What wouldn't be a bad idea would be buying a Ryzen 5 3600 and then going with a Zen2+ Ryzen 9 when they're out as your upgrade but honestly IMO and in my current position it's worth waiting to see how it plays out because it might be worth replacing your memory for Zen2+ as well because latencies and memory speed ideals may differ. In any case, buying cheap RAM for the 3600 today wouldn't make for bad spares down the road.

If you need a PC right now I'd buy but I wouldn't go HEDT or buy a Ryzen 9 just yet.

What does this MEAN on practice? Do those modern computers really add some sort of added ms due too many cores?

Im on an old ass q6600 10 year old Intel computer and im looking forward to get a 3900x, however im having second thoughts because this "latency" talk doesnt sound good on paper.