A SSL (Secure Sockets Layer) allows a web browser and a web server to communicate securely. It first authenticates the website to make sure they are who they say they are and then encrypts any information sent from the website.
Most shoppers now recognize as a secure browser. The lock in the top URL or the lower right of the browser lets them know that you are keeping their personal information secure and that they can trust you. Reinforce that trust by flashing your trust symbols.
Credit card gateways, like Authorize.net, will only accept encrypted data, making it required for anyone doing eCommerce. If you're transferring a website from another hosting company you can request that they export your SSL. It can then be transfered to our server which may save you some expense depending on how long your SSL is good for.