As mentionned above self signed certificate will generate an error on browser unless the the certificate in "imported" on the computer, if not done by the systeml admin it gives an unprofessional experience for the user ( most browser message will say something like site dangerous/unsecure)
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Let's encrypt is a free trusted certificate authority (aka CA) for most of the browsers meaning if let's encrypt says that your certificates are valide then most of the browser will trust your certicifacts and instead of a warning they will show a green lock on the address bar = the website is secure.