A maternal vaccine, sometimes also called a “pregnancy vaccine” (though that’s not strictly accurate, as noted below) or a “parental vaccine,” is a vaccine given during pregnancy not explicitly for the sake of protecting the pregnant individual so much as for protecting the fetus they’re carrying. The idea is that the pregnant person makes antibodies in response to the vaccine which are then transferred to the fetus across the placenta, providing the fetus with passive immunity to that disease that lasts for several months after birth.
The Tdap (tetanus-diphtheria-acellular pertussis) vaccine is recommended as a pregnancy vaccine, administered during the third trimester, so that infants have protection against pertussis from the time they’re born until at least their first or second DTaP doses. The most recently recommended pregnancy vaccine is the respiratory syncytial virus (RSV) maternal vaccine, also recommended in the late third trimester to ensure the baby has some protection against RSV from birth. Parents have the option, however, to get the RSV vaccine during pregnancy or to give their newborns an RSV monoclonal antibody drug (nirsevimab or clesrovimab) to receive passive immunity protection against RSV.
Two other vaccines recommended during pregnancy are the influenza vaccine and the COVID-19 vaccine. These are pregnancy vaccines, but they aren’t necessarily maternal vaccines since the purpose of their recommendations is to protect both the mother and the embryo or fetus, not just to provide passive immunity to the fetus (like the RSV and Tdap ones).