Presumption is basically counting on forgiveness from God even when you aren't sorry, or plan on "being sorry" later. The Catechism of the Catholic Church says, "2092 There are two kinds of presumption.Either man presumes upon his own capacities, (hoping to be able to save himself without help from on high), or he presumes upon God’s almighty power or his mercy (hoping to obtain his forgiveness without conversion and glory without merit)."
The first kind of presumption, relying on oneself for salvation, is definitely "cruising for a bruising" and will end with you going down in flames spiritually before long. I want to talk about the second one because it is easier to fall into and equally dangerous.
A good example of the second type of presumption would be consciously committing a mortal sin and justifying it by saying, "I'll just go to confession later." This is easy to do, and something that I have done before even without realizing it. However, it is very dangerous to the spiritual life. For instance, what if someone died in a car crash before they could make it to confession and "be sorry?" Not only is there this risk, but presumption misuses God's mercy. How would you like it if someone broke your laptop but told you, "It's ok, I'll just ask you to forgive me after I break it." Rather, if someone sins, there should be immediate repentance.
An important distinction to make though, is if someone is truly sorry. The church teaches that if someone dies with mortal sin before reaching confession, they are forgiven because they were truly repentant.
Some of the greatest saints even struggled with presumption. One of St. Augustine's quotes from his teenage years is, "Grant me chastity and continence Lord, but not yet." This is an example of hoping to be forgiven, but at the same time not wanting to let go of the sin and counting on God's future mercy.
While keeping this in mind, we must never doubt the infinite mercy of God. As long as we are sincerely repentant and don't play games with God's mercy, he will always forgive us. Even if we intentionally break someone's laptop.
Sources:
Catechism of the Catholic Church