Hi hun, welcome to the site! Overs and unders refer to where the implant is placed – over the muscle or under the muscle. People tend to have unders when they have very little breast tissue to cover the implant so they need the muscle to cover it. I think unders can be more painful and take longer to heal, but can look more natural and less chance if rippling etc.
I’m sure the post op girls will be able to go into more detail
xx