Mutations are alterations in the DNA sequence of an organism. They occur in the cell all the time, resulting due to various reasons. Most mutations that develop naturally have no major consequences on the organism.
The overriding reason why mutations that develop naturally don’t have any effect is because cells have highly sophisticated repair mechanisms that fix mutations very quickly before they have time to affect any changes or become permanent.
Another reason why naturally-developing mutations don’t have any effect is that most mutations are somatic. They mostly occur in somatic cells such as skin and muscle cells. These mutations have a limited scope of action and can only affect the cell where the mutation occurred and cells that arise from that cell.
On the other hand, germline mutations, which are mutations that occur in germline cells, eggs and sperm, have larger effects. This is because they are present in every cell developing from the affected egg or sperm, which pass the mutation on to offspring.