This option mimics the physical controls on a cassette tape player. This is necessary to load software into certain computer system. As you can see, there are all the standard buttons found on a tape player and the tape images will work exacly as they did on the real hardware.
There are also a couple of keyboard shortcuts that you can use to manage tapes without opening this screen. "F2" will start the tape player and "Left-Shift + F2" will stop it. If you are running a system that has control of the keyboard then you will need to press the "Scroll Lock" key to enable the UI keys for MAME before using these shortcuts.
The tape player will load the software at the same speed as the original hardware expects. You can speed up the process by enabling turbo mode for the machine by pressing the "F10" key. This will speed up loading from minutes to seconds. Just remember to turn it off once the software has loaded.