Starting Thursday, Gmail will always use an encrypted HTTPS connection for sending and receiving email.
Gmail has supported HTTPS since 2004 (an option to enable HTTPS as the default setting for a particular account was added in 2008) and in 2010, Google made HTTPS connections the default user option.
Now, Gmail will only transfer data between your computer and Gmail's servers through an HTTPS connection. That means there is no chance of someone disabling the option on your account or machine in an attempt to intercept data before it hits Google's servers.
Google also announced on its Gmail blog that it will encrypt messages as they move internally through Google's systems. In the past, although information sent from your computer to Gmail was encrypted, that same data wasn't encrypted when it went to another Google server or data center.
Starting Thursday, that changes. In its post, Google says that this change is "something we made a top priority after last summer's revelations."