Assistive Technology refers to the process (or specific device) that is utilized by students in special education to access the curriculum in their school.
Assistive technology is a broad area of educational technology - ranging from devices that cost only a few cents to a few thousand dollars. All of these devices are used by the student to participate in the school's functions (academic, social, functional, communicative, etc.)
A piece of assistive technology which many non-disabled adults use regularly is a pencil grip. Usually made of rubber or foam, these devices help many people to hold a pencil with the proper grip and technique, while also improving the ergonomics and comfort of the user. These devices are also used for students in special education to teach proper hand formation and improve writing accuracy.
A more advanced form of AT (and arguably the most commonly recognized type of AT) are communication boards. These boards have changed greatly over the past few years, but can generally be defined as hardware and software which allow a student to press a button and the device produces spoken words on the student's behalf. For instance, a child may be able to press a picture of a carton of milk and the device would then speak "MILK."
For a good introduction to the world of AT in Kansas, check out this YouTube video.