Peroxisomes contain enzymes that degrade fatty acids and amino acids. In doing so they produce hydrogen peroxide.
Hydrogen peroxide is very toxic because it is unstable and spontanously degrades to produce compounds called free radicals. Free radicals are very reactive because they have unpaired electrons and will react with a variety of cellular macromolecles and alter their structure.
Fortunately peroxisomes contain the enzyme called catalase that degrades hydrogen peroxide to the less dangerous oxygen and water.