Your site has received fraudulent donations or attempts at credit card transactions that are from spammers.
CAPTCHA (Completely Automated Public Turing test to Tell Computers and Humans Apart) is a technique used to distinguish between humans and computers. CAPTCHA is mainly used as a security check to ensure only human users can pass through. Generally, computers or bots are not capable of solving a captcha.
To enable CAPTCHA on your donation forms, goto Website -> Settings. Change the default setting from None to Text or Graphic. This will effect all credit card forms on your website.