A nucleic acid sequence is a succession of letters that indicate the order of nucleotide forming alleles within a DNA. The sequence of nucleotide tells what protein is made of. So a single change in the nucleotide makes a change in the protein and causes a malfunction.
A change in these nucleotide can end up making some part of the protein different. A single nucleotide change can change a single amino acid (amino acids are the building blocks of proteins) and the amino acid stops working at all.