A https is a security feature that helps protect data from being snooped by third parties, such as in public wifi hotspots. It is used maily by big organisations such as banks, work places, selling and buying sites etc. that handle people's personal and sensitive data such as full names, social security number or TIN numbers, passport and other forms of ID, date of birth etc.
Over the last few months, Gmail has been researching the security/latency tradeoff and decided that turning https on for everyone was the right thing to do.
Gmail annouced that it is currently rolling out default https for everyone. If you've previously set your own https preference from Gmail Settings, nothing will change for your account.
Problems with https, it can make your mail slower. How? Since encrypted data doesn't travel across the web as quickly as unencrypted data, using a https will render it slower than just a normal http.
Now that Google is making it a default feature, What if I don't want to use the https?
If you trust the security of your network and don't want default https turned on for performance reasons, login to your Gmail account and turn this feature off choosing "Don't always use https" from the 'Settings' menu (this applies to Google Apps users whose admins have not already defaulted their entire domains to https). Gmail will continue to encrypt the login page to protect your password.
NOTE: If you use offline Gmail over http currently, the switch to https is likely to cause some problems, click here to learn more about this known issue and how to work around it.