You might see a new broken lock icon in Gmail messages starting today. It can warn you if you're sending a message to an email service that doesn't support encryption or if you received a message that wasn't encrypted. Although Gmail itself supports email authentication and encrypting emails in transit, not all email services do, hence the new warning icon. If you see it when composing your message, make sure your email doesn't contain sensitive information. (But if you're sending sensitive info over email, you should encrypt it yourself anyway.)
Also, if you get an email from a sender who can't be authenticated, Gmail will replace the sender's avatar with a question mark icon, so you can be more careful when replying to the sender or clicking on links in the message.
TL;DR: These two new icons are security warnings, so just be a bit more careful when you see them.
Making email safer for you [Official Gmail Blog]