An SSL certificate is what enables HTTPS: a small file installed on a server that lets it encrypt data traveling between itself and a visitor's browser, so anything typed into a form, a password, a card number, can't be read by anyone intercepting the connection in between. Despite the name, most certificates today actually run on TLS, SSL's modern successor, though "SSL certificate" has stuck as the common term regardless.
For SEO, HTTPS has been a confirmed, if modest, ranking signal since Google announced it back in 2014, but the bigger practical impact runs through trust and browser behavior rather than the ranking boost itself: modern browsers actively flag any page without a valid certificate as "not secure" directly in the address bar, which is enough to make plenty of visitors abandon a site outright, especially anywhere a form is involved. Certificates need periodic renewal (or automated renewal through services like Let's Encrypt, which made free, automated SSL broadly accessible) and, done carelessly during a migration, a missing or misconfigured certificate on even part of a site can trigger mixed-content warnings that undermine the padlock's reassurance on every page, not just the affected one.
