An IdentCard was digitally encoded with a person's personal information. These cards could be scanned, allowing the viewer access to all of said information.
Behind the scenes
The only time IdentCards appear is in the episode "Safe" when Derrial Book produces one. Malcolm Reynolds and Zoë Alleyne Washburne produce only papers in "Safe"—nothing digital—it may be that they don't have anything of that sort. IdentCards were later mentioned in Six-Shooters & Spaceships, a supplement to the Serenity Role Playing Game.