It’s the difference between venial sin vs mortal sin.
Venial sins do NOT affect our relationship with God and we are not punished for them. They might extend our time in purgatory (because we would have more things to be purged from) but we are not punished for them.
Mortal Sins are the sins we are punished for.
Now in my post, I said “objectively speaking.” Grave sins (also informally known as “mortal sins”) are always grave matter.
However, if we don’t know a grave sin is a grave sin, it does not subjectively become a mortal sin. Why? Because God does not hold us eternally accountable for what we truly didn’t know was sinful.
I pray I’m a little clearer.
God bless