libssl-dev is the OpenSSL general library
SSL, also known as Secure Socket Layer, is a protocol proposed by Netscape for the transmission of sensitive data.
SSL uses a private key to encrypt transmitted data to prevent eavesdropping.
The most common use of SSL is for use with browsers, but many other applications can use SSL as well.
Generally speaking, URLs that use SSL start with https.
OpenSSL is a widely used commercial grade SSL tool.