Trying to understand time dilation (physics-tards)

Btw I've done a lot of research on this before posting. I'm just having trouble understanding how this works.

Like the movie "interstellar" where he was only gone for a few years but on earth it was near 100 or so years that passed.

Is this relative? Like say a person travels from a different galaxy towards earth, will time be dilated for them as well? Or does it have to do with earths gravity that affects light a certain way?