What are macros in a word document?

In text editors (such as Word, or other editors like VIM) a macro is a recorded sequence of commands that can be played back at any time. Say you wanted to do some fairly complex task many times in a word document (search for a word, cut and paste it to the end of the line, and make the whole line bold, or something weird like that). If you wanted to do this 100 times, going through the same sequence of key presses and mouse clicks would be a pain. Instead, you can tell Word to start recording a macro, do the task once, finish recording, and play it back in a single click as many times as you like. Now, if you really want to understand what's going on there, what Word is really doing is building a VBA program and executing that code. Because Word is not particularly secure and can be hooked into Windows at a fairly low level, people can write malicious Word macros to try and harm your computer. These are Word macro viruses, which you may have heard of. So be wary of using Word macros from people/places you don't trust.