Flex is a language, Flash compiler, and SDK that allows developers to create Rich Internet Applications (RIAs). It uses a combination of MXML and ActionScript and compiles code into SWF files to be used on the web or desktop via Adobe AIR. Flex gained popularity as an alternative to Flash due to its improved development speed, lower costs, and aesthetic advantages. The document encourages learning Flex through doing, thinking, experimenting, and keeping track of progress rather than just reading.