Frameshift mutations such as the substitution or deletion of a base can alter the entire DNA base sequence after the point where the base was substituted/deleted, this it causes the triplets to code for different amino acids which in turn results in the sequence of amino acids changing.
As the primary structure (amino acid sequence) changes, this causes the secondary and hence tertiary structure to change as well, causing the protein to become dysfunctional.
In comparison to point mutations which include silent, inversion, etc mutations, these mutations may cause only a single amino acid to either stay the same or change as a tiny region of the DNA base sequence is altered slightly, unlike frameshift mutations which may cause a huge portion of the DNA base sequence to be altered.