Sakuga is basically where animators go from using cheats like only animating the mouth or skipping frames to animating every frame with fluid movement.
Sakuga (作画) (lit., "drawing pictures") is a term used in anime to describe moments in a show or movie when the quality of the animation improves drastically, typically for the sake of making a dramatic point or enlivening the action....
The other end of this spectrum, however, is when the animation becomes exceptionally expressive and fluid -- when every single frame is animated, and the movements themselves are closely-observed and realistic (or, failing that, spectacular to look at). This is what's known as sakuga. Action-oriented shows tend to have the most instances of sakuga, but there are many examples of dramatic shows using them as well -- for instance, to highlight an exceptionally emotional moment.
Opening and closing sequences of shows typically feature sakuga (which sometimes leads to jokes about that being where the majority of the animation budget is spent, especially if the rest of the show isn't as fluid). ThoughtCo
There's more information about it here and here.
"Good sakuga" seems to basically refer to sakuga animation that looks exceptionally fluid or realistic, or just looks cool. It's seems to be subjective, based more on what people think looks good than on any scale or mix of factors.
Pages like this give examples of what they think of as good sakuga, and from reading the descriptions you can see they talk about what would be referred to as cinematography in live action shows.
