This module has two inputs: a signal that is being sampled, and a trigger input that indicates when the first input should be sampled. When a trigger is received, the current voltage at the first input is sampled (measured) and held (stored), and presented at the output. This stable voltage is held until a new trigger is received.
Sample and holds are most often associated with creating stepped random voltages. To do this, noise is fed to the main input; whenever a trigger is received, the voltage present at that input is some random value, which is then dutifully sent to the output.