X-rays are the second to last type of electromagnetic radiation from weakest (extremely low frequency radiation) to strongest (gamma radiation). It has a frequency between 30 petahertz and 30 exahertz.
On the series it is most commonly referred to when a doctor takes an X-ray image of an object or patient. This is mostly shortened to simply taking an X-ray. Here they send X-rays through an object be it living or not and the difference in density and materials through the object cause different levels of absorption of the radiation. In a human this is best shown on the images by white spots where the bones and any dense objects are in the body while the regular tissues barely absorb any radiation as the radiation just passes through it. X-rays can't penetrate lead.
Times it was used:
On Cassandra Fraiser when a heart murmur was heard. (SG1: "Singularity")
Dr. Janet Frasier and the other medical personnel were going to perform one on Alebran before he died.(SG1: "Metamorphosis")