Clicking a button or link that says "I agree" will let you enter a porn website under whatever conditions that you have "agreed" to (e.g. you are 18 or older).
It is supposed to be a deterrent to prevent minors from accessing pornography, often using scary-sounding words like "attest" and "perjury". But, like in the example below, it's rarely effective.
16-year-old: "Yeah, yeah whatever. So if I click 'I agree' I get in right? Sweet!..." (Clicks on button)