In a hashtable, a "bucket" is just a fancy name for an index of the array. They are also commonly known as "slots." Other than that "AuthenticOwl" nicely sums up how a hashtable works.
EDIT: And as "AuthenticOwl" mentioned, summing the ASCII values of a string is usually not an ideal approach to hashing as there is a high risk of collision. Also, if you want to understand how the code works, take it line by line.