Her seed.
What is Satan’s seed? Angels cannot reproduce.
Her seed refers to not only her Son, Jesus Christ, but also to His Church, his Mystical Body, of which He is the head. Jesus Himself refers many times to the reality of the Mystical Body of Christ, He is the vine, we are the branches, He is the head, we are the members, He is the Root of Jesse, etc....
Easy peasy to understand for those who have Faith.
The context of that passage from Genesis makes it quite clear who is the woman/her. It was talking about EVE. Satan has been at enmity with the human race since the human race began. It is Jesus - the God/man who will ultimately crush and put an end to the devil. This all started way before Mary was even born.