"the second category of point mutations includes nucleotide-pair insertions or deletions. these can result in frameshift mutations. what does this mean? "

Nucleotide-pair insertions or deletions happen in such a way that an extra base pair is added, or an existing pair is removed. 

Insertions happen when in a given sequence, another base pair is added resulting in a change in the sequence of amino acids produced. This renders the sequence useless because of the mutation. 

Deletions happen when one base pair has been removed and as a result of that, the sequencing process changes. 

Because there is a prescribed pattern for sequences, an insertion or deletion in a particular structure can alter the amino acids next in line for production resulting to complications.