How much detail do you want?
Short version:
Mocap we're all generally familiar with, someone wearing a suit with sensors that that acts something out, then that data is mapped to a digital character. Not often used in animated films but has been before (Tin Tin being the most famous example, the animated Final Fantasy film is another), very common for video game animation and live action VFX work.
It's also far less automatic than the behind the scenes videos would have you believe and still requires tons of manual work to get the motion looking like it has the correct weight, timing, and details. Go watch something like the LOTR Gollum clips comparing the Andy Serkis mocap to the final animation side by side and you'll see tons of little details that the animators added in.
Keyframe animation is the process of setting a "key" on a certain frame number, a digital character pose that can be everything from hips and legs to fingers and eyelids. Another key is then set on a subsequent frame, and the animator adjusts the curves between the two keys to get the right timing and easing between them. If you've ever seen animation that looked really floaty or weightless, it's usually because the animator didn't set the correct interpolation between frames.
All that said, it's not especially important in this context. Most any Disney/Pixar/Sony film is going to be largely if not completely animated by hand. I'm sure there's some smaller direct to video Netflix type animated films that have used mocap, but I'm not aware of any major animation studio using it outside maybe some for facial animation.