Presumption means you presume on God’s mercy - take it too much for granted.
One example might be if you consider committing a mortal sin and before even committing it say ‘it’s OK, I’ll commit this sin, then go to confession straight afterwards’. God’s mercy doesn’t work that way, and the person, clearly doesn’t have a proper attitude to forgiveness if they are taking it so much for granted that they will be forgiven.
Another might be the idea, held in some non-Catholic circles, of OSAS (once saved always saved), in other words that once one is ‘saved’ by baptism, sinners’ prayer or the like that one can sin in any manner one wishes afterwards and it will not make a difference to one’s salvation.