When I register into an account in a web site I receive a note to ensure that I am not a ROBOT . Does this happen in real and how ?

They mean prove that you are a real human NOT an automated program used for illegal processes like hacking.

It prove that you are a human and not an computer program 

it save the web site from let down becouse the high traffic on it

Robot here means a a computer that can fill out information automatically without human aid. This is used to prevent a computer from creating thousands of accounts on that website to spam other people.

Register into an page register 

down user and password you got regisery and forget password choice register

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