When you sign in to Yahoo!, you’re now protected with Secure Socket Layer (SSL) encryption. This means that your password is more secure in transit when you sign in. However, with SSL encryption, pages take longer to download. Some users therefore choose to sign in using Standard mode to achieve quicker response time. Signing in from this page uses our Standard mode of encryption. This doesn’t use a Secure Socket Layer (SSL) form of encryption. It’s also made available for browsers that do not SSL. Your password is still being passed to Yahoo! securely with this option, albeit without SSL.
Note: SSL isn’t supported by all browsers. To take advantage of submits over SSL sign-in, you might need to upgrade your browser to the latest version. [source]