SSL stands for Secure Sockets Layer and is a cryptographic protocol that provides a communication security over a computer network.
You can learn about it in more detail here, but in layman’s terms and when it comes to websites this basically means that visitor’s communication with the website is encrypted.
This is especially key if any concrete data is being submitted by the visitor via website. So any kind of webform is already a strong indication that having the SSL certificate on your site would be advised. Of course, having a webshop where your customer’s credit card details are being communicated, SSL certificate becomes a must.