How to layer/create negative padding effect

I'm using an index page and would like to layer my second page into the header. Anyone know code for how to do so?? It's a common practice I see online so perhaps I'm using the wrong vocabulary but it looks like they do negative padding.

Here is a link to a website that does this: https://modernadventure.com/our-trips/yucatan-claudette-zepeda-2015clz0101/