Chrome says “This page contains some resources that are not secure” in all https websites

If you simply click on the lock symbol next to the struck through "https://", it will tell you what it means.

EDIT:

If you click on the lock symbol, it opens an information window that tells you about the encryption type of your connection to the website and about the giver of the certificate signature. It also tells you when you first visited the website.

It is nothing to worry about most of the time. Many of the web services and applications on the Internet in these days, are not 100% self sufficient and a web page, in today's Internet, is nothing but a combination of many different items from different locations. In order words, because the web is getting closer and closer to be an inseparable part of our social lives, it needs to incorporate many social items as well. Doing so, requires a website to try and retrieve data from different sources, which not all of them are using SSL protocol. This causes that green lock to go gray.

In a simpler language, when a webpage, which is using HTTPS(SSL), tries to incorporate or use some other items (Pictures, scripts, xml or css files, etc.) on other websites or servers, that do not use https, your green lock turns gray. Most of the time there is nothing to worry about, firstly because there is nothing much you can do, and secondly because this usually happens on non-so-highly-sensitive web pages, and you should only worry when it happens on you PayPal or bank account page.