Base stats, as known as species stats is hidden stats and indicates the three of the following:
Attack (associated with CP)
Defense (associated with CP)
Stamina (associated with HP)
Pokemon's CP is based on two of base stats, attack and defense. HP is decided by stamina.
Example)
Attack & CP Dragonite has the highest attack base stats at 250. That means Dragonite's CP is possible to be higher than that of other species.
Stamina & HP Chansey has the highest stamina at 500, and so its HP can become very high. However, its attack and defense is low relative to other species. As a result Chansey gets lower CP relative to other species.