To proliferate means to choose any number of permanents and/or players that have a counter, then give each one additional counter of each kind that permanent or player already has
It sticks around and pays you back your investment
In technical terms, one could say that Wizards of the Coast combines the two cognitive frames of our understanding of magic in Final Fantasy and magic (meaning sorcery and instant spells, for the meticulous MTG nerds among you) in Magic
This makes flanking excellent for removing 1/1 creatures that would otherwise chump-block your attacker, as well as killing those X/1 deathtouch creatures before they can deal any damage
The curious thing to note is that those packs are from prior sets, so you could get five from Winterspell, or you could end up with some classics
A late-game Star Charts also digs very deep, giving you tons of selection over what you draw