How it's done ? (Linkin Park Website)

Hey everyone, im kinda new to all that web stuff and coding and recently i went to linkin park's website and there was an effect i'd like to recreate and see how it's made, I think it's called "hover" but when you move your mouse, it transition to another picture, and how do we do to have that fluid look ?

here is the link to the website : https://linkinpark.com/