An RLSA is a remarketing campaign but on search not display. The above video shows how you can apply a remarketing list to an adgroup, you can either have the campaign search and RLSA or just RLSA by selecting “target and bid” or bid only.
The idea behind the RLSA is, as the visitor has visited your site before, the ad will be more relevant to the user than the other ads. You can then make your targeting slightly more broad and it will still be relevant.
I have seen RLSAs work very well with bid adjustments and in their own campaign, so it is well worth testing.