Mutations are the changes in the DNA sequences which may occur due to error in DNA copying or any chemical that may result in copying of DNA accurately. The mutations may occur in somatic cells or in germ-cell. A mutation in germ cell is capable of passing on to the next generation.
Some Mutation can be beneficial and some may be harmful in nature.
Mutations may result in formation of different proteins that will change the phenotype of the organism. If this new phenotype is capable of adding advantage to the species' survival then this mutation will proliferate and pass on to next generation. Hence, such mutations are beneficial and results in greater variation.
Some mutation can be harmful to the organism. Like, a mutation in tumor suppressor cells may result in the cells to divide continuously without any check on it. This may result in cancerous situation and may be lethal to the individual. Therefore, such mutations are harmful.
In nutshell, mutation in the DNA sequence can be beneficial or harmful depending on the type and place of mutation in the DNA sequence.