They are known as Holotags. You can get them from BOS bodies or some other places in the Wasteland. You can trade them for caps to Scribe Jameson in the Archives Room of the A-Ring at Citadel (Brotherhood of Steel DC Headquarters, which is actually The Pentagon ruins). You need to reach certain point of the main storyline in order to get access to Citadel. I think you also get XP in addition to caps for each Holotag you turn in.