What is SSL?
A deprecated cryptographic protocol that secured network traffic before TLS replaced it. The name persists colloquially.
SSL (Secure Sockets Layer) was the original encryption protocol for the web, introduced by Netscape in 1995. SSL 1.0 was never released, SSL 2.0 shipped briefly, SSL 3.0 dominated the early web. By 2014 every SSL version had been broken (POODLE killed SSL 3.0 in particular) and the industry moved on.
TLS (Transport Layer Security) replaced SSL through versions 1.0 (1999), 1.1 (2006), 1.2 (2008), and 1.3 (2018). TLS 1.0 and 1.1 are themselves now deprecated; modern stacks default to TLS 1.2 and 1.3.
In casual usage "SSL" still means "the encryption that makes a URL HTTPS." When someone says "install an SSL cert," they really mean a TLS certificate. The terminology has stuck because changing it would break too many docs, products, and habits.