Most of us would not go on a long hike in a brand-new pair of boots. You first want to put a few casual miles on them to soften the material and make sure they perform well. This preliminary effort can help you avoid a lot of misery out on the trail. If you think of a spacewalk as the ultimate hike (who doesn't?), then it's easy to understand why spacesuits undergo the same type of break-in process before they're ever sent into space.
About the Suit
Before getting into the specifics of how spacesuits are broken-in, a little background on the suit is warranted. The NASA suit that astronauts have used for spacewalks since the dawn of the space shuttle era is the Extra-Vehicular Mobility Unit (EMU). The EMU is a modular design comprised of a handful of interchangeable subcomponents (helmet, upper torso, lower arms, gloves, etc.). Many of the various subcomponents that make up the suit are available in multiple sizes.
When an astronaut gets sized for an EMU, they do not get a dedicated suit to call their own. Rather, the product of the arduous sizing process is a chart illustrating the specific subcomponent sizes which provide the best fit for that astronaut. Whenever the astronaut needs a suit for a training event or mission, technicians reference the chart to pull the appropriate hardware off the shelf and assemble a correctly-sized EMU. The suit is torn down after the event and the individual subcomponents are placed back into inventory.
Over time, worn-out subcomponents get retired and replacements are manufactured. This new hardware undergoes rigorous inspection and testing before it can be added to the inventory. Yet, even more must be done before these EMU bits are used on an astronaut's suit.
New EMU subcomponents are required to undergo a break-in process called "cycling". Whereas factory testing is typically performed using only the individual subcomponent, cycling introduces the piece into a complete EMU. The intent of this effort is to begin softening the stiff layers of new fabric and to verify that the part performs properly in all respects. This is done by exercising the hardware with repetitive, spacewalk-inspired motions. For those who participate in cycling events, the term "exercising" is particularly appropriate.