Hello,
I saw recently the option "animation curves" and i don't know how to use and what is this tool for? In the user guide , instructions aren't very clear and. So, is that a curve animation tool like in adobe animate? How to use it? I tried to search on the internet but without results.
Thanks.

The Animation Curves docker is for adjusting/manipulating the path between defined points that control the opacity of frames along the timeline.
The defining points are Opacity Keyframes and these are markers that are placed on existing frames.
In the Animation docker, the icon above the onion skin icon has a drop down list with 'Add opacity keyframe ' as an option. This makes the current frame an opacity keyframe, as well as whatever else it is.

https://docs.krita.org/en/reference_man ... curve.html

In the Animation Curves docker, you can drag the opacity value of an opacity keyframe up and down between 0 and 255. You can move it left and right to move it into a different frame. You can add more opacity keyframes, etc with the icons along the top. You can also adjust the shape of the curve between opacity keyframes.

The end result is an opacity transition graph that will be applied to the layer over the length of the animation. This is useful for various fade-in and fade-out effects as I'm sure you can imagine.

The Animation Curves docker itself needs to be quite big to be useful (I find) and the scaling is a bit clumsy. I've no idea why it goes to negative values (needs a bug report) and it takes a while to set it up to a useful size and arrangement. It does work though
The best way to learn how to use it it to play with it and see what happens.

If you want more detailed and highly customised opacity control, you can animate a transparency mask. This must have an animated parent layer to initially make it animated. After that, you can duplicate it and drag it onto other animated or non-animated paint layers.
You can convert an animated paint layer into an animated transparency mask if you like, but you can't convert an animated transparency mask to an animated paint layer.