Macros allow you to perform multiple commands by only using 1 button. Macros can make your life much easier if you often use certain abilities one after the other regularly. For example a popular rogue macro would be to use Cold Blood and then immediately Eviscerate. Reducing a 2 click/button press down to 1.

Essentially macros are just simple lines of scripting code which are performed in order. Most macros are quite simple, but some macros can be quite complex. More complex wow macros usually comprise of some sort of situational test, for example: If you are in combat, the macro will vanish, or if you are out of combat, the macro will stealth.