The website of StatenClinic is now secured by a SSL certificate. The gray padlock indicates:

• The website's address has been verified, proving that the domain is not being spoofed.
• The connection between your browser and Statenkliniek.nl is encrypted to prevent eavesdropping.

When you click on the padlock, it tells you that you are actually connected to Statenkliniek.nl as certified by Comodo. It also assures you that the connection is encrypted so no one can eavesdrop on the connection and steal your data being entered e.g. on the form for appointments.