Sefer Tiferes Shmuel quotes the Vorka Rebbe zy”a as explaining the words “this is the thing (davar) that Hashem has commanded” to mean that every word (dibbur) that a person says creates a commandment from Hashem. In other words, if a person says that he is going to do a certain good deed, it becomes a “commandment of Hashem”, and he is now obligated to do what he said as if Hashem commanded him to do it.
This teaches us how powerful one’s words truly are.