I overheard my dad telling my mom that she needs to go for an HSG test soon. What exactly is an HSG test?

Yesterday, I was in my room getting ready for school and I overheard my mom and dad in the hallway. My dad kept telling her that she needs to go to the doctors for an HSG test soon. My mom then agreed, but said that she wanted to wait another month. So, what exactly is an HSG test? My mom seems to have allergies a lot, so is it like an allergy test or something? I'm a 15 year old guy, by the way. Thanks!!