A silent mutation has no effect on protein structure and thus also on protein function. It occurs when mutations affect a section on the DNA that does not carry a blueprint for a protein (non-coding DNA).
Another possibility is that the change in the base sequence nevertheless leads to the correct amino acid. Indeed, because the genetic code is redundant, there are usually multiple code words for an amino acid. For example, the base sequences CCU and CCG both provide for the formation of the amino acid proline. You can easily read this from the code sun.