Nearly all samplers have the ability to play a sample, or a portion of a sample, in a continuous loop such that the sampled sound can be sustained indefinitely. (Generally, an envelope from an envelope generator is then applied, effectively replacing the sampled sound's "natural" envelope.) In order to do this, there has to be a point where the sampler reaches the end of the segment of the sample to be repeated and then jumps back to the beginning of the segment. The issue is that the the start and end points of the loop do not match up properly (according to the loop mode being used), there will be an audible pop, click or thump every time the loop repeats.
For most ordinary loop modes, where the loop is being played forward (or backward) continuously, it is necessary to choose loop points where the waveform is near the axis (near to zero volts), in order to avoid clicks and pops. Additionally, the portion of the waveform immediately preceding or following the loop point can effect how clean the transition is, and large timbral differences between the two ends of the loop can produce an effect that is distracting or annoying as the loop repeats. So the ability to choose proper loop points is part of the art of building and playing sample-based patches. Since the late 1980s, most samplers have had the ability for the programmer to "scrub", or play the sample at varying speeds back and forth across the transition while adjusting the loop point, until the transition produces the desired audible characteristic. More recent samplers usually have built-in aids that can choose loop points automatically, or present the programmer with a list of suggested loop points that produce clean transitions.