in easy terms (for beginners)

TL;DR

XML parsing (or XML deserialization) ==> input: valid XML, output: data structures

XML serialization ==> input: data structures, output: valid XML

XML parsing (a.k.a XML de-serialization)

You take a .xml file (example.xml) as input to process it with your programming language of choise, so that your programm can do something usefull with the data in that file. Your programm will transform the information from the file into data structures that your programming language can deal with (i.e. lists, arrays, objects, etc.).

XML serialization

Your programm (in any programming language), transforms information represented as data structures (lists, arrays, objects, etc.) into a valid XML output which can be saved into a file or tranmitted to another programm.

NOTE: Technically the input (when we are takling about parsing) and the output (when we are talking about serialization) does not have to be a file. As said in the more professional answer above it can be any input/output stream, too. And files don't have to have .xml extension, they can have any file extension which represents a valid XML format (i.e. .svg is also a XML based format). The key to understanding is, that when we do XML parsing we have valid XML on the input side and data structures on the output side, and when we do XML serialization we have data structures on the input side and valid XML on the output side.

To give an example from the Python world: you can use buildin packages (like xml.etree.ElementTree) or third party libraries (like lxml (recommended) or xmltodict) to do both - parse (deserialize) or create (serialize) XML data.