About alias

[Sam Hu]

I am confused again this time about the key word alias when I read below tutorial:
http://www.dsource.org/projects/tutorials/wiki/MetaBinarySearch
Let's say this one:
template bSearch(alias Match, alias Fail, alias x, A...) {
    const bSearch = bSearchAlg!(0,Match,Fail,x,A);
}

what is the meaning of the 'alias ' before parm Match in the parms list,and what's the purpose?

Thanks and best regards,
Sam