It’s a biblical concept- a generational curse can be anything negative that is passed on until someone breaks the cycle eg like alcoholism in families or an abuse cycle.
Beyonce would most probably be referring to Meghan as being a catalyst for change brought in to disrupt some of the darker/negative aspects of the family - as part of God’s plan for her life and to bring more light into the Royal Family.
This is common parlance and platitudes that give ‘purpose to the suffering’ In evangelical Christianity, which Beyoncé would be familiar with and I assume Meghan is using here to validate some of what she has done/gone through.
Personally, I am not making a comment on its truth.