Along with shoes, Giro saw the need for better insoles. They so liked the results of their insole design work that they're selling them as an after-market accessory, the SuperNatural Fit Kit. Their goals were to make their insoles more comfortable for a wide range of foot types, and that they'd offer enough adjustability to perhaps negate the need for investing in custom insoles.
One of the few benefits of the wafer-thin insole that many shoe companies supply with shoes is that it is wafer thin. No bulk in what is an extremely close-fitting shoe is most often a good thing. This keeps the ball of the foot very close to the sole and in turn the pedal spindle, minimizing stack height, and it keeps the heel very low in the heel cup, maximizing the cup's ability to hold the heel down. Giro started with these two benefits and added one of the benefits of custom insoles; arch support. Arch support, is critical for cycling comfort. When the leg pushes down on the foot, the arch can flex, which can lead to shoe movement, which can lead to alignment issues, to the foot moving in the shoe, to developing hot spots.
Another benefit of custom and semi-custom insoles is a "metatarsal button" that provides greater mid-foot comfort. Giro has included a subtle button as they've found wide variation in individual rider's ability to find such a feature comfortable.
The base of the insole is a thin layer of EVA foam. It's thick enough that normal use of the insoles will help to customize, or tune, or crush some of the EVA so that you'll see divots in the foam after a few rides. This will increase the surface area of contact points, which should increase comfort, but the EVA is secondary to the three arch supports that come with each insole.
There is a small, medium, and large arch. They can be easily and quickly swapped in and out. The small maintains the contour of the insole itself. The medium adds 3mm to the arch compared to the small. The large adds another 3mm beyond the medium. These are easy to use. First, in either bare or thinly-socked feet, place the support under your foot. Try different sizes until you find one that seems snug against your foot. You want it close because you don't want your arch acting like a leaf-spring each pedal stroke. Figure out the size. Ride. Adjust if necessary.
The insoles are covered with X-Static fabric. The name is familiar because you've already seen Giro and others use it in helmet pads. It's a silver, the mineral, fiber that is bonded to a polyester fabric to make the fabric anti-microbial, anti-stink.
You'll see that each size of the Giro SuperNatural Fit Kit fits a number of size shoes. These are "trim to fit" insoles and there's a cutting guide underneath each insole, but your best bet is to either compare it to your stock insole or shove it inside your shoe before commencing on any alterations.
I have a pair of Fizik R3 shoes that fit quite well, but I was having foot pain on longer rides. My feet are pretty normal, if a touch wide, but my arches have gotten fairly high after a few years of wearing low-support/minimal shoes. I dropped these insoles in, with the highest-support attachment, since that's what I use in my Giro mountain shoes. I rode about 50 miles in them today, with virtually no foot pain.
The arch-size inserts attach using a double velcro, and I've never had it slip in my mountain shoes, and don't expect it to in these. You can trim them to fit fairly easily by matching your shoes' insole to the Giro version, drawing a line, and getting after it with some scissors.