What is causing the self-signed certificate error in the Vue apollo client's operation?

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The self-signed certificate error in Vue Apollo client's operation is caused by a security feature in browsers that checks the validity of SSL/TLS certificates which authenticates HTTPS connections. The error occurs when a website's SSL/TLS certificate is self-signed rather than being issued by a trusted Certificate Authority. Browsers do not accept self-signed certificates as valid and will throw an error.