A mutation is an accidental change in the DNA sequence of an organism. As you know, your DNA is arranged in chromosomes, which is a long sequence of DNA nucleotides. A gene is a subsequence of DNA nucleotides within the longer sequence making up the chromosome. A mutation arises when the sequence gets changed either by an error during DNA replication or by some other accident (including exposure to certain chemicals or forms of radiation, for instance). The reason a gene is able to serve as a code (stored information) is because of its particular sequence of DNA nucleotides.
Mutation changes that sequence. Therefore the code is changed, and when that altered code is used to make RNA, the RNA will also be altered. Similarly, when the code in that altered (mutated) version of RNA is used to build a protein, that protein might be different from the unmutated version.
Since proteins directly determine phenotypes, a mutant protein (resulting from a mutant form of DNA for the corresponding gene) might result in an altered phenotype. Mutation is the original source of genetic variation, and it is the reason why there are different species and why there are differences between individuals of the same species. As you continue to study genetics, try to keep in mind that differences in genotypes and phenotypes are ultimately due to mutations that have accidentally occurred.
After you watch these videos, make sure you can define mutation and explain how mutations, such as that in sickle cell, can result in a changed phenotype.