Thin client is a networked display, keyboard and mouse, and the session you appear to be running on it is really on a remote system which sees the display, keyboard, and mouse virtualised. Some thin clients also include audio in/out, camera, usb ports (and in days past, also serial ports), which again are all virtualised across a network from the back-end system which is really running your session.
In general, thin clients are stateless - there's nothing stored on them (in disk or memory) that is necessary for your session, so switching them off, or swapping them if they break won't interrupt your session which will reappear exactly as you left it when you reconnect from the same or a different thin client. 'Thin' means the system is presenting a thin venier with the display, keyboard, and mouse, but there's nothing else there - the real system you are using is elsewhere.
In addition to dedicated thin client terminals such as Sun Ray and Wyse which were used in the past, there is thin client software such as Citrix, VNC, etc which you can run on Raspberry Pi (2+ or later), Linux, and even on Windows.
X Windows is another example of thin client when running the clients (applications) on a different system than the X display server. NCD X Terminal servers used in the early days of X were thin clients, although the term 'thin client' hadn't been invented back then.
There are a number of different reasons people use thin clients. In the office I'm currently working, the "desktop" PC's are all in racks in a data centre 40 miles away with maintenance teams to keep them working, UPSs, failover to a different datacentre if something goes wrong, etc. The desktop thin clients have no data on them and are all identical, which makes running, maintenance, patching easy, and if it dies, it's swapped out in minutes and I'm back up and running again without even being logged out of my real desktop which has datacentre reliability. I can also access it identically from home or on the move, using thin client software on a Raspberry Pi, laptop, or home desktop system. If the thin client is stolen, there's no data on it (I can't write to anything on it). Many thin clients are only about
5 watts max too.
You might look to run a browser in kiosk mode (comes up automatically as the only application, possibly without any login depending on your security requirements.)