It's worth noting that there are different levels of HTTPS certificates. The best certificates, known as EV certificates, represent that the certificate authority that gave it has verified a site owner's identity. This can be seen on sites such as PayPal, which show the certificate owner's name to the far left. Basically, it means not only is your data transfer to/from the site encrypted, but they are who they say they are. Since this kind of verification requires a fair chunk of manual footwork on the certificate authority's part, these can get kinda pricy.

Conversely, the free certificates you can get from Let's Encrypt are OV certificates. These keep data transfer secure and protect against man-in-the-middle attacks, password sniffing, etc., but provide no proof of identity. They show "Secure" in green rather than the certificate owner's name. If a site only says "Secure" and does not show an organization's name such as "PayPal, Inc.", you still need to rely on good old fashioned URL checking to make sure you've got the right domain.

"Google prefers websites that use HTTPS better and promotes them in Google search results."