PayPal Client ID and Secret Key is required, when PayPal is integrated in websites as payment method.
These credentials will be provided in REST API configuration for REST API to work with your PayPal Account.
Here are the steps to get Client ID & Secret Key:
1. Login to PayPal & Click on Applications
Go to PayPal Developers Homepage: http://developer.paypal.com/
2. Create New Application
Click on Create App to create new application.
3. Fill in New App name And create App
4. Click on App name you provided to get REST API Details
5. Get Client ID & Secret Key for Sandbox(Testing) and Live PayPal