The blade is curved in a unique waved shape and resembles a kris or a keris dagger. There are many legends or beliefs surrounding the Kris/Keris blade, and one of those includes the idea that "the blade gives complete invulnerability to its owner."
In Red Hood: Lost Days, this dagger is given to Jason as a gift by Talia al Ghul. She tells him it is a replica of one her father used to carry. We also learn in Under the Hood that the blade is extremely sharp.
The dagger Jason carries is both iconic and symbolic of a weapon that was not likely to be found on the streets of Gotham City by many. It also further ties him to the al Ghul family.