We have a custom to nullify our vows on erev Rosh Hashana. Chayei Adam 138 explains that we do this to save us from the punishment coming to us by not keeping to our vows. We have already been warned by the Torah in Parshas Ki Tetzai 23:22 not to take upon ourselves anything in the form of a neder. (only b’li neder) Anything which has been said as a neder, if not kept to, is a disappointment to Hashem for which we must bare responsibility. (See Ramban). There are different explanations as to what our punishment for not keeping to our word will be. Kli Yakar explains that if any money was saved by not keeping our neder, the financial gain will be taken away from us some other way.
As not to go into the day of judgment with Hashem being disappointed in us, we annul all of our previous vows. Derech Hachaim brings from the Shebalai Haleket 317 that the general annulment only works for vows we do not remember. If we remember them we must specify them to at last one of the three judges. Many are of the opinion that this only applies if we specified that we want to do something. If however something became a neder because we did something without saying or thinking that this is b’li neder, we do not need to specify the neder.
It is important to note that for the hataras nedarim to work, you must understand what you are saying. If one has trouble understanding the hebrew text, you should instead say it in english, or at least look over the translation of the words not understood beforehand.

Rabbi Tzvi Aryeh Hyman, Mir Yeshiva
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