Customization is to write your own code or override existing rules in Pega to meet the requirements.
Configuration is to use existing OOTB rules in the application to meet requirements.
The main difference is that customization involves modifying a rule or writing a custom java code to achieve a specific requirement that is not available OOTB. Anything which involves custom code will raise guardrail warnings as this is not a best practice and not recommended. Customization is more risky as this may break during upgrade.Configuration involves achieving requirements using OOTB rules which is less risky and can be done in less time less effort and less risk. Configuration will make a stable application and easily upgradable.
Customization meant to enhance the existing / provided functionality according to the user requirements.
Configure means to use the existing tools to achive your requiremnt. ( For example you have a requirment to utilize the rest-service which is already avaialble, then you can use the existing connect rest wizard inorder to connect that service and use this utility either in actvity or data-pages wherever its required)