WhatsApp app as you know is based on your phone number and verifies it by sending a security code to your number which you have to enter in your WhatsApp app to confirm your number but people who have access to your phone can easily exploit it for opening your WhatsApp account on their phones reading all your conversations.
But every problem eventually comes with a solution and this is just what WhatsApp did.
They Added A Two-Step Verification For Verifying Your Ownership Of The Account
How to enable it?
To enable the two-step verification all you need to do is go to Settings > Account > Two-step verification, then tap Enable and set your code.
With everything else WhatsApp also allows you to add an email address to your account but it does not get confirmed so be sure to enter it correctly.