What is mean by key frames in html?

Answer 1

it's animation; kind of like function () in JS.
How it works:
.element {
animation: name 1s infinite;
}
- @keyframes name{ 50% {
height: 0;
height: 50px;
}
}

So you name the @keyframes, inside it you put what you want to happen(animation);
im this case, the width transitions to 50px from 0; in one second.

The animation won't happen unless you call it with an element

.element{

animation-name: name;
animation-duration: 1s;
animation-iteraction-count: infinite;

}
I can't really go into details.
It's better if you look it up:
Google,
W3School,
CSS-Tricks.
--