In most situations it is not desired that capturing and displaying the input signals starts at random moments, like e.g. immediately after the previous measurement, or when the user presses start. Most of the time it is preferred that capturing and displaying starts when one or more input signals meet some predefined conditions. We call this triggering. To let an instrument trigger at the desired conditions, the instruments are equipped with a trigger circuit.
To get detailed information on triggering, read the library article about triggering.