A vast majority of the traffic on the internet is served in a secure way using certificates to both encrypt what we send over the internet, as well as verify the identity of a website. Nearly every website you visit on the internet has its own certificate that tells you that the website you are visiting is the real deal, and is not being impersonated. But just having a certificate for a website only goes so far, because anyone can create a certificate saying "I am this website!" Which is why we have Root Certificate Authorities, a special set of certificates that are used to sign certificates saying "This certificate that claims to be this website can be trusted in its claims." It's like a permission slip from a parent, the child can say you gave permission to go on the field trip, but without a signature from a trusted authority (the parents) it shouldn't be inherently trusted.
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