In simple: if the websites are without SSL then the web requests are sent as plain text but with SSL it will be in an encrypted format.
Imagine you are running an e-commerce website, where your customers enter their credit/debit card details into your shopping site and the details send to the server as plain text, the details can be retrieved by a middleman(a hacker)
If there are SSL enabled, then the details are in an encrypted format and are hard to be hacked.