what's meaning of "(a) =>"

[step8 via Digitalmars-d-learn]

I'm trying to study D programming
Following is code from vibe's example(web_ajax) code:
~~~~
void getDataFiltered(Fields field, string value)
{
	auto table = users.filter!((a) => value.length==0 || 
a[field]==value)().array();
        render!("createTable.dt", table)();
}
~~~~
I can't understand the expression　"(a) =>"，there is no defination 
of "a",how does this work?
thanks for help