A substitution is a type of mutation in which there is an exchange between two bases (for example, switching A for G). Changing a base can lead to a different codon, and consequently, a change in the protein produced.
Sometimes, the mutation can be silent, meaning that the substitution didn't change the codon or the protein.
However sometimes they may change to a "stop" codon and because of that the protein ends incomplete. If it affects an important protein it can completely change the organism.