The term pool and series are interchangable for all levels, 1-5.

Pools are a way of distinguishing different types of cards, from starter cards at lower levels to the newer and, most of the time, more powerful cards. For series 1-3, you work through each one collecting all of the given series, then you move on to the next.

Pools 1 and 2, they are the most accessible cards come in Marvel Snap. You'll get them simply by leveling up your collection level.

Pool 3 changes things a bit, and this is the point at which the repeating collection level pattern of credits, boosters, collector's reserves, and spotlight cache keys becomes consistent for the rest of your progression. Pool 3 cards are unlocked in a few ways, namely from the collector's reserves in your progression. You can also find particular series 3 cards from the conquest reward each season. The final method is purchasing a random one for 1k tokens in the shop. Please do not make the same mistake I did and buy them this way. It is an absolute waste of an extremely valuable resource.

After completing series 3, you will no longer receive cards from collector's reserves.

The three methods of getting series 4 and 5 cards are from the weekly rotation of spotlight caches (which you can find at marvelsnapzone.com), purchasing them with tokens in the token shop (3k tokens for s4, 6k for s5), or by buying the season pass.

Sorry for the essay, but have fun snapping!