SSL certificates are used to secure transmission of data through web sites. Transmission of such data as credit card details, account usernames and passwords, any other sensitive information has to be secured to prevent eavesdropping. SSL certificates can secure not only data transmitted through web sites. Email transmission can be secured by SSL as well.
With SSL certificate data is encrypted prior to transmitting through internet. Encrypted data can be decoded only by server to which you actually send it. This is a guarantee that information you submit to web sites will not be stolen or tampered.
Starting from 06/08/2014 Google announced that having a SSL certificate installed on your web site will increase your ranking position, which is another great reason to use SSL. Certificate itself is a big cryptic code.