All it does is turn a list of space separated ids into a jquery filter that would match those ids. The replace just means with an aria-describedby="myid myotherid anotherid" we want a jquery selector similar to the below. 

`errors.filter("label[for=name], #myid, #myotherid, #anotherid");`

The replacement simply adds the necessary verbitage between each id. :)