SSL certificates is your way of knowing that the site you are browsing is encrypted.
Look for the padlock icon.
If your website is not encrypted Google will take a dim view of your site and warn users that the website is unsafe or malicious. This is a massive over-reaction. You do however need to see the padlock when you are buying things with your PayPal or credit card. Many sites are static and do not perform transactions. Google does not care. It wants all websites to have an SSL certificate.
Up until now I have successfully employed a free certificate as this is all my clients need. Lately I have noticed that these certificates are not being recognised as safe as they are self-signed as in not authenticated by a CA [certificate authority].
I have upgraded to a professional and paid service using COMODO. Clients who are needing this level of SSL certificate can take this option.
COMODO 5 year SSL certificates
$110.00 inc gst