The PNG image format allows for information that is not nesaccerily relevant to the image itself. For example, some textual information might be stored in the iTXt, tEXt, and zTXt chunks. However, these text chunks do not define the image itself and can be safely deleted, and thus to reduce the file size.
For more information about PNG chunks, read PNG Specification, Version 1.2
PNG Optimizer removes all unnecessary chunks to reduce the image size without affecting the image quality.