A CS2 sticker capsule works like a small case for stickers. You open it and receive one sticker from that capsule's list, with rarer finishes such as Holo, Foil and Gold appearing less often than the paper version. Some capsules need no key, while some older ones did.
The most traded capsules are tied to Majors. Each Major gets capsules for the teams, the players and sometimes the event logo, and those capsules are sold in the game only while the tournament is on. When the sale ends, the only supply is what players already hold.
Why do Major sticker capsules go up in price?
Supply stops, but demand does not. People open capsules over time to get stickers for crafts, and every opened capsule leaves the market for good. If the team on the sticker becomes popular later, or a player on it becomes famous, demand rises against a shrinking supply. Early Major capsules are the clearest example of this pattern.
Not every capsule rises. Some events have so many capsules sold that supply stays high for years, and capsules for teams nobody remembers can stay cheap. Buying capsules is a bet on future interest, not a sure thing.
Should you open or hold a sticker capsule?
Opening gives you one random sticker, and the average value of that sticker is often below the capsule's own price once the capsule has become collectable. Holding keeps the capsule sealed and tradable. Many investors hold sealed capsules and only open when a specific sticker inside is worth more than the capsule.
Capsules are also part of a Major's other items, such as Souvenir packages and viewer drops, so their prices often move together around the event. Watch the whole set rather than one capsule alone.