A terminal emulator is a software application that replicates the functionalities of classic computer terminals. These terminals consisted of a monitor and a keyboard, and were used mainly to access another computer, such as a minicomputer or a mainframe. The terminal emulator performs the same functionality in software.
Techopedia explains Terminal Emulator
A terminal emulator allows a host computer to access another computer, including remote ones, through either a command-line interface or a graphical one. The communication is made possible using protocols such as Telnet and SSH.
The terminal emulator allows the host computer to use or run applications on the remote computer, as well as transfer files between the two. The two systems need not be running the same operating system.