An SSL Certificate is a digital certificate issued for a domain by a central authority called the Certificate Authority. To be issued an SSL Certificate, you must purchase an SSL Certificate and then go through a verification process conducted by the Certificate Authority.
An SSL Certificate does 2 things: a. Encrypt the information sent from your user's browser to your website b. Authenticate your website's identity.
By doing these 2 things, an SSL Certificate protects your customers and in turn increases their trust in your online business. This is especially important if your website requires users to login using passwords or enter sensitive information such as credit card details.
Further to a secure connection with your clients-visitors or that you protect your data from hackers or ....etc. you get a very high rank from all search engines such as Google, Bing, Yahoo ....
Last year Google change it's search algorithm to give much reasults first from safe websites with SSL than to other simple sites.