Metaphysics refers to a person's understanding of and belief concerning the basic set-up of reality.
For example, which side a person assumes is real between the religious view of the universe being essentially purposive and meaningful - or, alternatively, the modern mainstream assumption that the universe is some mixture of deterministic causes and random events (e.g. some mixture of relativity and quantum theory).
Metaphysics affects everything, sooner or later; because it frames all legitimate questions and dictate the legitimacy of answers.
Mainstream, normal, modern morality is is based on the assumption that everything which happens is either caused or random - and therefore ultimately meaningless and purposeless. By this account, all morality is therefore necessarily and only a matter of feelings; and is indeed a 'projection' of our own subjective feelings onto an uncomprehending and dead universe. Thus (according to mainstream modern metaphysics) things are good or bad, ultimately, because they make us feel good or bad - and this is universalized into a 'moral system' along the (dubious) lines that we feel better or worse when we feel that other people feel better or worse.
(Yes, I am aware that this is not actually 'a morality' - but that is what passes for morality in the public arena - see After Virtue, by Alasdair MacIntyre.).
There is no possibility of genuine moral debate within this modern secular framework, which is why we never see any. We see only assertion and counter assertion of feelings, and the imputation of feelings to various others.
For a Christian this makes honest 'casual' discussion about moral issues (around the water cooler, as it were) impossible - because all casual discussion is non-Christian, indeed anti-Christian, in its basis and tendency. To converse honestly about morality, the modern Christian must define and contrasts his metaphysics - which hardly makes for casual 'water cooler' conversation!
When an 'issue' arises in everyday conversation - such as whether tattoos are a good thing (or, some specific person's particular tattoo), or whether somebody's divorce was a good or bad thing and why, or what should be government policy about uncontrolled mass immigration, or whether some friend has behaved well or badly...Well, the honest Christian is faced with reinforcing, tacitly by participation without clarification, a moral system he deplores - or saying nothing, which dishonest but antisocial.
But I can well recall, when I was an atheist, how absurd it seemed to me to envisage 'a moral universe' - I thought of stars and planets and volcanoes and I could not imagine how they could be part of a moral plan - and since that was most of the universe, then objective morality seemed absurd. Likewise with 'love' - how could love be the most important things (as Christians assert) is most of the universe is dead and non-conscious? At most I could imagine that the whole universe was a means to the end of the Christian God's purposes for humans - but that seemed like an absurd overkill when comparing the size of the universe with the tiny planet earth with its scattering of people.
I was, in sum, trapped by my metaphysical assumptions concerning the nature of reality - and I was trying to 'insert' God into this ready-made modern secular 'universe' (which had, as a matter of historical fact, been constructed so as to render God at best superfluous, and indeed to mainly and specifically to eliminate Him).
It is a great liberation of spirit and source of energy and hope to identify, challenge, recognize the ridiculousness of, then discard the nihilistic metaphysics which we had inculcated by mainstream, secular, modern society - via the mass media, the systems of education, politics, law, by work and bureaucracies... To see that this metaphysics is arbitrary, absurd, unnatural, self-refuting.
And then to realize that I have the power, right and autonomy to replace it with something that is spontaneous, natural and Christian - and to discover (despite my knowledge being radically incomplete and imperfect) that then, suddenly, everything makes sense!