Need help understanding Lumen Gentium

Hey y'all. So I was debating with a protestant about Mary's title as Mediatrix, and doing research to defend my arguments I came across this passage from Lumen Gentium:

"Just as the priesthood of Christ is shared in various ways both by the ministers and by the faithful, and as the one goodness of God is really communicated in different ways to His creatures, so also the unique mediation of the Redeemer does not exclude but rather gives rise to a manifold cooperation which is but a sharing in this one source."

My question is, how should this be interpreted? I can kind of see how it relates to Mary, but does that sharing in Christ's mediation apply to all Catholics, or just to a few? And what does that sharing entail?