Verifying your payment source is simply adding a payment source to your Freelancer.com account. If you so choose, you can verify your PayPal or Skrill account, or credit card. This can be done through the Payments & Financials section of your Settings page.
You can also click your balance from your Dashboard and select Verify Payment Method.
To learn more about how to add a payment source to your account, you can read this article: How to Verify Your Payment Method.
Custom Charge Verification
Custom charge verification is the process of verifying or authenticating your credit card on your Freelancer.com account. This is done through the Trust & Verification section of your Settings page and ensures your ownership of the card and, thus, the safety of your account.
To learn more about verifying your credit card and detailed steps on how you can do this on your account, you can read this article: How to Authenticate Your Credit Card.