Rosh Hashanah is also known as the day of judgement, when we go in front of the King of Kings the be judged and to receive decrees for the coming year. We all want to come prepared to the trial and to have a good defense, to achieve this there are many halachot (laws) and traditions that will give us positive points (as if) to sweeten our
decrees.
The Arizal delivered a whole set of things to be done to come fully prepared to the judgement day. The starting point is Hatarat Nedarim to do (nullification of vows); according to the simple understanding, one has to go in front of a Bet Din (rabbinical court formed by three persons) and say th words that can be found in most of the sidurim or in any Rosh
Hashanah machszor. In a deeper way, Hatarat Nedarim also erases curses, ayn hara (evil eye), witchcraft and any external negative force for the future.
The Arizal also brings the Pydion Nefesh, he brings a very specific way of doing Pydion Nefesh, we separate certain amount of money...