Autoit is a bunch of tools that compile/run scripts written in the autoit syntax (basicly a scripting/programming language).
People can release there scripts in a compiled form (.exe) which doesnt require the autoit tools to be installed on users computers, but this raises questions on wether its a virus or not and they prefer to release there source instead.
Its worth downloading the tools from:
http://www.autoitscript.com/autoit3/downloads.shtml
So you can compile and run autoit scripts or make your own.
Thas as basic as i can explain sorry, hope it helps.