What is ment by genetic mutation?

A mutation is any change to a strand of DNA which is passed on to an organism's offspring.

DNA mutations are caused usually by chemical interactions or radiation (natural and manmade), and most are harmless.

The only ones which cause serious defects (that I know of) are changes to sperm or egg cells, stem cells (which might ultimately be responsible for the DNA and development of an entire region of your anatomy) and changes which cause cancer.

If a fetus or embryo is exposed to intense radiation, they will be at high-risk for harmful genetic mutations because their body is still developing, whereas an adult will be relatively immune since most of the time if a strand of DNA in a cell is damaged the cell just dies and is replaced.