It is a letter which you decorate by cutting the paper into patterns and send to someone before Easter. You then add a short verse of poetry (kind of like Americans do for Valentine's Day) and sign it with as many dots as the letters of your name. If the one you send it to guesses that it is from you, you owe them a chocolate Easter egg and if they don't, they owe you one, when you reveal that the letter was from you.
Many of the letters will contain a pressed snowdrop flower and the poem will often be about the snowdrop, one of the first flowers to bloom in the spring, or about springtime and may contain a hint of the name of the sender.
Children began sending these to adults in the 1930's in hopes of getting Easter eggs and chocolate. Today, they are mainly sent by children who make them at school. You can also buy ready made snowdrop letters at the post office and bookstores. The handmade ones are really the best though. This is a wonderful tradition in Denmark.