The tramp took a step forward and struck the table with his fist. “Now I am going to tell you, Mr. Ironmaster, how things are,” he said. “This whole world is nothing but a big rat trap. All the good things that are offered you are nothing but cheese rinds and bits of pork, set out to drag a poor fellow into trouble. And if the sheriff comes now and locks me up for this, then you, Mr. Ironmaster, must remember that a day may come when you yourself may want to get a big piece of pork, and then you will get caught in the trap.”
Lagerlöf's "The Rattrap" is a metaphor for how humanity is beguiled by the "traps" of life, "all the good things that are offered." The rat trap seller makes his way through life by selling rat traps and supplementing his income through the hospitality of people along the way and through the stray kronor that he might steal from this or that person. For instance, the story opens with the rat seller accepting hospitality from a crofter--a small tenant farmer--from whom he later takes 30 kronor after breaking one of his window panes to be able to reach the kronor pouch.
When the rat trap seller happens into a blscksmith forge at Christmas time to warm himself at the billowing forge fire, he is mistaken by the ironmaster for an old friend from days gone by. The ironmaster insists that the other come home with him for Christmas festivities. The other refuses as, of course, he knows he is not Captain von Ståhle for whom he is mistaken.
The ironmaster sends his daughter to persuade the man in the forge and, of course, she is successful. It is then discovered that the rat trap seller is only a rat trap seller and not the Captain. The ironmaster threatens him with the sheriff and opens the door for him to leave. This is when the rat trap seller delivers his speech that spells out the meaning and metaphor of the story: good things of life are traps; they become "cheese rinds and bits of pork" to tempt us, and none are exempt from the temptation.
The sheriff is left in peace; the daughter closes the door her father is holding open and insists the man stay through Christmas. He sleeps. He is lavished with good food, new clothes, a comfortable bed. In the end, he is so overwhelmed with the daughter's kindness that he rises above his constraints in life and makes amends for his wrongdoing. He leaves the 30 kronor for her to return to the crofter. He gives her a rat trap that continues the metaphor and signs himself as the mistaken identity, Captain von Ståhle.
The rat trap is a Christmas present from a rat who would have been caught in this world’s rat trap if he had not been raised to captain, because in that way he got power to clear himself.