what is this only secure content is displayed. what's the risk?

The primary site (in the address bar, which has 'https' prefix) is encrypted.

The browser (when left in stock settings) will accept and display items not from that 'https' server, but from unencrypted servers; usually advertising.

Your settings do not allow unencrypted content to be displayed, and so the notice.

It should be noted that many primary sites have co-mingled their 'https' content with 'https' advertising content, in a maneuver to circumvent those who specify that the browser display "only secure content".

Yahoo has made this blend, and if you want their sites to function completely, you must accept the 'secure' ads.

(That has security implications beyond the scope of the question.)

What browser used, and in some cases what add-on's you have may affect the interplay of those things, and site interaction.

This may include ad blockers, such as "AdBlock Plus"; try turning it off on Yahoo to see if that makes a diff.

If you mean answers to Yahoo questions then it's an Adblocker if you are using one. You cannot put answers up on Yahoo if have an adblocker turned on. And Secure Content is Displayed only means they are showing you Secure content only and just click Show the Lot [if comes up] Other than that means nothing and doesn't affect nothing.

If you're using IE (internet explorer) this will happen...Go download chrome to prevent this from happening.

Hope I could help!