A screen reader is any application that tries to read what is on a computer monitor using sound or another device. More specifically, a screen reader attempts to identify and interpret data that is being sent as output for display. Once the data is interpreted, it is presented to the user in some way. For instance a simulated voice, a Braille output device, or text-to-speech software.
Screen readers are quite helpful to those who are visually impaired, blind, learning disabled, or unable to read. Common examples include Narrator for Windows, VoiceOver for macOS, and ChromeVox for Chrome OS.