The Sample and Hold circuit uses two buffers to keep a voltage level stored in a capacitor. Pressing Ssample will charge the capacitor to the present signal level, while the input buffer ensures the signal won't be changed by the charging process. From there, the output buffer will make sure that the voltage level across the storage cap won't decrease over time.
Pressing Sclear will short out the storage cap, discharging it and setting the output to 0V.
We can build the Sample and Hold circuit with mechanical pushbutton switches to see it in action. In actual practice, the switches used are various forms of transistor switch, which provides cleaner switching and also allows another circuit to control the sample and clearing operations.
Excellent Sample and Hold circuits like the LF398 are available on a single chip for cheap and easy use.
Sample and Hold circuits are used internally in Analog to Digital conversion. We might also use them to hold a given signal value from any particular sensor on a robot, for analysis and later use, especially if we don't want to bother encoding it and sending it to the CPU's memory