What are pttys?

Pseudo-teletypes (pttys, ptys, other variant abbreviations) are
pseudo-devices that have two parts: the master side, which can be
thought of as the `user', and the slave side, which behaves like a
standard tty device.

They exist in order to provide a means to emulate the behaviour of a
serial terminal under the control of a program. For example,
telnet uses a pseudo-terminal on the remote system; the remote
login shell sees the behaviour it expects from a tty device, but the
master side of the pseudo-terminal is being controlled by a daemon that
forwards all data over the network. They are also used by programs such
as xterm, expect, script, screen,
emacs, and many others.