When you take care of your banking online, your bank’s website uses a protocol called https to secure your financial data.  If you login to unencrypted wireless networks, i.e. at coffee shops or airports, https will keep your email encrypted so someone sharing the network can’t read it. If you use Gmail, you have the option to always leave https on to keep your email protected each time you access it.  By default this option isn’t selected so you will need go to your Gmail settings and choose Always use https next to Browser Connection then click Save Changes.  Keep in mind that https may make Gmail a little slower; you can always turn this feature off if you are using a secure network.

Source – The Official Gmail Blog:  Making Security Easier