At the most basic level, jACT-R really only relies upon a Java virtual machine that supports version 1.8 or later. The system utilizes many open source projects such as Apache Commons, OSGi, Mozilla's Rhino, etc. In order to manage all of these dependencies, jACT-R also utilizes Eclipse, which is both an application platform and an IDE. Not only is the jACT-R IDE built on top of Eclipse, but Eclipse is also used to manage plugins, modules, other dependencies and launch behavior.
Do you need to use Eclipse to run jACT-R? No. You can run it from the command line, but currently only Eclipse based binary downloads are available and the build system is within Eclipse. If you want command line only, it's usually easier just to install from Eclipse and manually extract the jar files.
So what do you need to download and install before actually installing jACT-R?
Once they have been installed you can go on to the jACT-R installation.