Using https, also called http over tls, authenticates your web server and your website and protects against transactions being intercepted or read by a third party other than you as the website owner or your visitor.
The https also adds additional security when using public Wi-Fi spots, if the page is not encrypted possible packet-sniffing could be done, in short a visitors private information that they are willing to share with you could be stolen from a third party.
There are several provides of SSL certificates pending on your specific need. Comodo is well known name in SSL and have several options for SSL certificates.
If you need help in adding SSL to your website contact The WebMaster E today and we will get you all setup.