Strange include file

I’ve downloaded a fla file from another site and it refers to an actionscript include file called

I’m sure this is well known thing but I’ve never seen it or heard of it. My flash throws an error message every time I try to publish it saying “File Not Found”, however it still publishes the movie but it don’t work.

Can someone explain the signifigance of this well known file (that I’ve heard of…kinda) and how I fix this problem please, thanks.