the easy explaination would be "something endless"... but it's also not as easy as that.
think of it as super complicated math only to create some kind of geometric shape that's infinite... looping forever... endless...
let me give you a example.
that image was rendered using a software called "jwildfire" CHECK IT OUT HERE
this is a full resolution 4k image so sorry if it took a bit to load.
as you can see in that image you actually can't see... the end of it of course. its a symmetrical shape that's repeating itself to infinity.
pro tip: open that image in a new tab or download it and zoom into it. it's not the same if you are only viewing it while scrolling past with your browser!