What is a Macro

Essentially macros are lines of script that execute in-game commands. They allow you to combine multiple actions into one user-customized button that you can use just like a spell or ability. For example, you can take a string of commands that you would ordinarily press one after the other, and combine them into one button that executes them all in sequence. These helpful tools have the ability to improve and streamline your entire play experience.

Macros have something to offer every type of player, casual gamers and power users alike, and macros suit a variety of play styles and situations, such as leading a PvE raid, battling in arena PvP, or simply role-playing in Ironforge. Macros can be one of the most useful tools in your arsenal for both PvE and PvP. They save time, consolidate multiple actions, aid communication, and overall make your time in Azeroth more convenient.

The above paragraphs were extracted from Blizzards Macro Guide. If your not familiar with the basics on how to create/use macros, this may be a good place to start.