Losing one nucleotide would cause the entire framework of codons (the 3 nucleotide set that codes for the amino acids) to move off by 1 which results in likely completely different amino acids being called for when the protein is built.
In the case of CF, the normal CFTR gene which codes for Cystic Fibrosis Transmembrane Receptor - a membrane protein, with a point mutation would not be made properly and is therefore not placed into the membrane of the cells of the offspring - this protein is very important in moving sodium into the cell - if this is messed up it results in disease.
Note that CF can result in a number of different types of mutations to the CFTR gene...a frameshift mutation is just one way.
