You can send a signal tone to your mobile phone which will be played back for two minutes. For the signal tone be played back, your mobile phone needs to be turned on and be connected to a mobile network or Wi-Fi network.
Click Lost Mode and follow the instructions on the screen to lock your mobile phone.
You can lock your mobile phone with a code and add a message to the phone screen saying that you've lost your mobile phone.
Click Erase iPhone and follow the instructions on the screen to delete all phone content. Please note that you will no longer be able to use Find My iPhone when you've deleted all phone content.
You can delete all phone content to prevent others from accessing it should your mobile phone get stolen. When you've deleted all content, your mobile phone needs to be activated using your Apple ID.