Users complaining of a "Select a certificate" popup on my web sites

I have a couple websites launched in Azure. The URLs end with .azurewebsites.net which is fine with me. But recently, we've received multiple complaints that when the users visit these sites, they receive a popup in their browsers similar to the following:

I don't know anything about https or SSL certificates. My question is:

1. Why did this start happening recently? I've had my websites up for a long time and now we're getting this?
2. Can I update my websites so that users don't receive this popup anymore? If so, how?

Thank you.

If you don't want to recieve the popup , you can disable the setting in Azure Portal Go to Azure Portal =>Your Web App =>Configuration =>General Settings

• Select Optional under Client certificate mode

OR

• If you don't want to disable , You can provide the windows sign-in as authentication