In the comic book and graphic novel industry, a splash page is a large image panel that can span a complete single page or, more commonly, two adjoining pages where they appear as one huge single panel. Designed to "make a splash", they're normally filled with lots of action or to set up the upcoming story or setting if they're at the beginning of the book.
Examples could be Superman punching the villain of the month so hard that they fly off the page and onto the next; having one ensemble team (like Charles Xavier's X-Men) facing off against Magneto's competing group on the next page, both crews scowling at the other and getting ready to rumble; to having a space ship barely escaping a supernova on the previous page.
Overall, they're meant to make the reader say "wow" from the amazing artwork. The downside is the pages must be set properly so they appear across from each other and not on opposite sides of the same page. Matching the art so they line up to give the appearance of a single huge panel can also be a bit tricky, especially since the pages were usually hand-drawn on bristol board and inked.