In seismic processing, "stacking" just means the summation of one or more seismic trace.
When the traces are summed, if there are events with the same amplitude at the same two way time value, then the amplitudes will combine together, and get stronger.
Events that are on one trace at a particular time, and not on another trace at the same time, will not get stronger.
You can use this as a method to detect weak, flat events that are swamped by other reflection signals and/or noise; this might be the "flat spot" cause by an oil-water contact, for example.
Exactly how this is implemented in the software you are using will vary - you'd need to check in the software manual or with the support desk to find out exactly what method(s) are available and how to employ them.