This warning is referred to by multiple places that discuss using Cisco IOS, which is an operating system designed to be run on devices that are Cisco-branded hardware. Technology, such as the hardware checking for signed code, may prevent that operating system from being used on other devices.
The information about using Cisco IOS is often rather specific to just that operating system. In other words, there are often details that are intended just for people who are using that operating system. These detils are not usually meant for general purpose use by hardware other than the Cisco-branded devices that were designed for professional use. Directions about using the software may assume some basic familiarity with Cisco equipment, like seeing output from a Cisco device and interacting with the Cisco IOS operating system.
Many people have completed formal training for using this equipment, such as Cisco Network Academy programs offered as part of college courses. People may spend months gaining training and practice in preparation of being able to obtain the Cisco Certified Network Associate (“CCNA”) Routing and Switching certification, which many people have considered to be Cisco's “entry level” industry certification.
Although the software is closed source, proprietary, and non-redistributable, Cisco IOS is rather significant because it has had some fairly widespread usage. It is designed only for equipment made by a company named Cisco, but that company has been rather prominent in selling equipment. Therefore, such details are useful for some people. However, if you are looking for something to do at home, and don't have any Cisco equipment, don't waste time trying to figure out how to get instructions to work.
The introduction to using Cisco equipment, and especially the Cisco IOS: Basic usage guide, may be resources that help to get started with using this type of Cisco-branded equipment. For example, the Cisco IOS Basic usage guide contains details about becoming familiar with using the “command line interface” (“CLI”) that is provided by some software called the “EXEC shell”, which is part of the operating system.
Some other operating systems, like the operating system built into Cisco ASA equipment, might not generally be categorized as the “IOS” operating system. The same might be true with a Pix's operating system, and perhaps CatOS and MX-OS as well. However, the operating systems may typically be similar enough that many details from the “Cisco IOS Basic usage guide” may be relevant enough that the guide will still be very helpful for these other operating systems. Other details about these other operating systems may be listed on the Cisco operating systems page.