A self-signed SSL certificate lets you run Apache over HTTPS on Ubuntu 26.04 without buying a certificate from a Certificate Authority, which makes it the practical choice for staging servers, internal apps, and local development.
Many Linux users run into this situation when setting up a new website or web app, as they want HTTPS working right away, even before getting a real trusted certificate.
A self-signed certificate encrypts the connection, but browsers will still show a warning because the certificate is not signed by a trusted authority, which is normal and does not mean the connection is unsafe.