A CS2 drop pool is Valve's list of what can drop to players. It is not one list but a set of them. The active drop pool holds the newest cases and collections and supplies most weekly drops. The rare drop pool holds older cases that still drop but far less often. Anything outside both is discontinued and no longer drops at all.
Valve changes the pool when it releases a new case or collection. The new item enters the active pool and something older usually moves down a level. Those moves are announced in update notes and are watched closely by traders.
How does the drop pool affect case prices?
A case in the active pool has steady new supply, so its price stays near the floor. When it moves to the rare pool, supply drops sharply, and the price starts to climb as people keep opening the cases already in circulation. When a case leaves the pool entirely, supply is fixed, and the price depends only on how fast people open them versus how many want to hold them.
This is why traders buy active-pool cases in bulk while they are cheap and wait. It can take years for a case to leave the pool, and there is no guarantee any given case will rise, but the pattern has held for many past cases.
Where can you see the current drop pool?
Valve does not publish a single page for the drop pool. The community keeps lists based on update notes, in-game files and observed drops. Check a recent, dated list before acting, since a case that was active last month may already have moved.
Skin collections work the same way as cases. A map collection in the active pool drops its skins in weekly care packages, and when it leaves the pool, the only new supply comes from trade-up contracts using skins that already exist.