A palm scanner is a security device designed by the Tau'ri to scan the handprint of the user, unlocking only if the handprint matches one in its database.
They are used to restrict access to an area or system.
After a Reetou was able to override and open the Stargate's iris by using a computer somewhere in the SGC, thus triggering a security breach, the palm scanner was permanently installed into Stargate Command's iris control on the recommendation of Captain Samantha Carter, the second-in-command of SG-1 who also personally oversaw the installation of the palm scanner into the Dialing computer. (SG1: "Show and Tell")
Since then, it has proved invaluable to defending the SGC as it serves as an easy and effective way to let the SGC teams through while also stopping any possible alien threats from gaining access to the SGC while personnel associated from the SGC who have also had their palms scanned into the device itself are the only ones allowed to use it.