HSG or hysterosalpingogram. Read on:
SOURCE
During a hysterosalpingogram, a dye (contrast material) is put through a thin tube that is put through the vagina and into the uterus. Because the uterus and the fallopian tubes are hooked together, the dye will flow into the fallopian tubes. Pictures are taken using a steady beam of X-ray (fluoroscopy) as the dye passes through the uterus and fallopian tubes. The pictures can show problems such as an injury or abnormal structure of the uterus or fallopian tubes, or a blockage that would prevent an egg moving through a fallopian tube to the uterus. A blockage also could prevent sperm from moving into a fallopian tube and joining (fertilizing) an egg. A hysterosalpingogram also may find problems on the inside of the uterus that prevent a fertilized egg from attaching (implanting) to the uterine wall.
Hope this helps.
BTW, it's great that you'll be having this test. For sure, this will help you have a baby real soon. Good luck hun!