Secure Sockets Layer (SSL) is a standard security technology for establishing an encrypted link between a server and a client—typically a web server (website) and a browser, or a mail server and a mail client (e.g., Outlook).
Any computer in between you and the server can see your credit card numbers, usernames and passwords, and other sensitive information if it is not encrypted with an SSL certificate. When an SSL certificate is used, the information becomes unreadable to everyone except for the server you are sending the information to.
The browser/server requests that the Web server identify itself. The Web server sends the browser/server a copy of its SSL certificate. ... The Web server sends back a digitally signed acknowledgement to start an SSL encrypted session. Encrypted data is shared between the browser/server and the Web server.
If you accept credit card information and store it in a database so you can process it using an offline POS machine or charge it manually on your merchant account website, then you definitely need an SSL certificate to secure the credit card data as it is transferred.