To do that they ask us to write some words that we can identify inside an image. That is an efective method but sometimes it is very difficult even to humans and allways irritating.
The security check appears when we are adding new friends or sending message to people who are not in your friend list.
To disable this check system in your account you have to verify your account with facebook, it needs you to enter your mobile number and it will send you the security code on your mobile number.
To get verified on facebook by follow this:
Click on the verify Your account link on the captcha popup (link circled red in the image below):
After that facebook will ask you to select your country and country code and then enter the cell phone number below it. Go ahead...
Once you click the submit button facebook will check your phone number and send you a message with a alphanumerical code that you have to enter in the next window.
After you press confirm and problem solved!
Have fun!
