Modern Amiga gaming relies on WHDLoad, a tool that installs cracked or original games to hard drive. Most WHDLoad slaves require Kickstart 3.1 (or 3.0) to be present in memory. The A1200 ROMs pack provides the necessary resident modules for these games to quit back to Workbench seamlessly.
This article provides a deep dive into what these ROM packs are, why you need them, the legal landscape surrounding them, and how to configure them for the best possible experience using WinUAE, RetroPie, or MiSTer.
Most users looking for a "ROM pack" are actually looking for
The ROM allows the computer to boot. To see a GUI, you need the Workbench floppy images (ADFs) or an installed hard drive file. You can find legal copies of Workbench 3.1 included with Amiga Forever.
For the games themselves (often referred to as ROMs in emulation circles), there are several well-known "packs": TOSEC: Commodore Amiga (2012-04-10) - Internet Archive
"A Blast from the Past: Exploring the Amiga 1200 ROMs Pack"