The Black Winter is a cosmic primordial force blah blah blah that exists basically to destroy the universe. Galactus, the only survivor of the previous universe, is the only being with a chance to beat the Black Winter. But his basic power level is grossly insufficient for the task. To get the energy he needs he must eat 5 specific planets that have a specific kind of energy. Thor is made aware of this, and won't let Big G just slaughter the populations, so he elects to act temporarily a a Herald to make sure that nobody dies. Big G charges him up.
Anyway, the Black Winter was coming, before Galactus was ready to face it, so Thor, fully empowered with Thor-ness, Odin-ness and Galactus-ness, unleashes hell against the Winter. And restrains it for 14 seconds. This force is explicitly presented as universe-busting, and significantly more powerful then Galactus, and it hasn't had any trouble destroying stuff unimpeded up until now.
That's about it, really. The arc is only 4 issues old, Thor has only been Ultra Instinct for 3, so, feats are light outside of the ones presented so far.
I agree, it doesn't make sense that a herald-level of power cosmic moves the needle compared to Thor and Odinforce together, but it's presented as a powerup for the guy, and, as we've seen with a couple of outliers, Galactus can give more or less power as he chooses, AND AND the PC sometimes seems to expand upon natural abilities. So PC amping someone who's already a skyfather might be a bigger deal then when it amps a normal dude.