Your website will need a certificate if you want to use TLS to encrypt traffic so that it cannot be intercepted and read -- or worse, modified -- as it flows from a browser to your picos and vice versa.
If you elect to use TLS, the browser will show your visitors that your website is "safe"* because your URLs will be able to use the HTTPS protocol, rather than the unencrypted HTTP protocol.
To use TLS, you need a certificate which offers proof that you control the web server which hosts your picos and hence your website. A certificate authority will issue you such a certificate after verifying that you do indeed control the web server.
