How often a card or rarity turns up when packs are opened, usually given per pack (for example 1 in 21 packs). A pull is any card you open from a booster pack.
For new players
Pull rates come from people counting large numbers of opened packs. For 30th Celebration, TCGplayer counted a Special Illustration Rare in about 4.88% of packs; Pokémon itself says it cannot guarantee a specific card in any pack.