Macbeth's conflict is intensified when Duncan names Malcolm as the "Prince of Cumberland", which means he is next in line for the throne.
When Macbeth says "that is a step / on which I must fall down or else o'erleap / for in my way it lies" he is saying that now Malcolm is in his way on his path for the throne and he now has to figure out a way to either jump over Malcolm or it will cause him to fall.
