An expander is a program that allows you to type in a form of shorthand. You create a collection of short abbreviations and longer expansions. Then you type, for instance, 49yoaaf, followed by the space bar or a particular "marker key", depending on how the expander works, and your computer replaces what you typed with an expansion of 49-year-old African-American female. You can set up expansions for words, phrases, and entire blocks of repetitive text. You can set up expansions for all the referring doctors' names and addresses, etc. In sophisticated expanders, you can set up expansions that generate sentences that pause at spots where you need to choose a variable word, or expansions that actually run short macros to do things like jump to a website and look up the word you just typed.
What an expander can do for a transcriptionist is to provide a saving of anything from 25% to 80% or more of the keystrokes needed to produce a document, depending on how good the expander is and how good a system the MT sets up for the expansions. The result is more money per hour and less wear and tear on the hands.
Many people use Word's AutoCorrect or WordPerfect's QuickCorrect as a make-do expander, and those will serve the purpose in a limited fashion if one is careful not to overload them with too many expansions. Investing in a true expander program, however, gives you a lot more power, a lot fewer problems, and is an investment that will pay for itself many, many times over. (The advertised maximum for entries in AutoCorrect is 7000, including the base typographical error correction entries it comes preloaded with. Experience has proved that it generally bogs down and/or corrupts and loses its entries long before that, and 7000 expansions is a drop in the bucket compared to what many MTs create. You also don't have options to use a marker key to avoid conflicts between standard English words and expansion shorts, to create separate glossaries or dictionaries of expansions for different clients or types of transcription, or to run commands from an expansion short, etc.)
In short, an expander is one of the things that separates the girls from the women; the typists from the transcriptionists.