Repeat or pulsing refers to repeating the weld and hold portion of a weld schedule. Normally there is a very short off period of one or two cycles with continued application of force between the two weld sequences. This gives you the ability to dissipate the heat energy a little further into the part away from the center of the weld nugget.
Pulsing is putting a modest amount of power into the part over a modest length of time and repeating it. The total adds up to a large amount of power over a longer period and it lets a larger area of the part be heated up. The result is that you can grow nuggets larger. It is very difficult to get a nugget as large as the electrode face diameter. Pulsing may make it possible. Sometimes it is possible to exceed the electrode face diameter with pulsing.
This method is a better method than over driving the system with very high current and power levels in one pulse. This pushes the shunts, cables electrodes and transformers to extreme currents and heat which leads to more maintenance than pulsing at lower power levels.
References: RWMA- Resistance Welding Manual 4th Edition