Customization is a process of making changes to a website as desired. The customization process is something that runs through the whole of the life cycle website time by time.
In some cases WordPress theme customization may be required because the theme used is not fulfilling the requirements exactly as required (functionally or design-wise). Or WordPress plugin customization may be required to alter, expand, or restrict any functionality of the plugin. On the other hand, there might be some customization stuff relating to the layouts and design as well, like WordPress front-end customization.
If you are looking for an experienced WordPress full-stack freelancer/developer for your WordPress theme customization or for WordPress plugin customization, then drop me an inquiry so we can have a discussion about your task.