Secure Sockets Layer (SSL) is a security protocol that encrypts the connection between a web browser or app and a server, so that passwords, card numbers and other data cannot be read or altered on the way. It was developed by Netscape in the mid-1990s and has since been replaced by its successor, Transport Layer Security (TLS), where the full explanation is.
Every version of SSL is now considered insecure; the last one, SSL 3.0, was formally deprecated in 2015, and modern browsers and payment systems accept only TLS. The old name survives in everyday speech, as in "SSL certificate" for the certificate a site uses to prove its identity, even though the connection actually runs on TLS. Both rely on cryptography, combining public-key methods to agree keys with fast symmetric encryption for the data itself.