Combat PowerCombat Power (CP) is essentially a Pokemon's Overall Level in Pokemon GO, virtually a calculation of its overall strength via its Base Stats and Individual Values. All Pokemon have one, from Wild Pokemon to Pokemon at given Gyms to your very own Pokemon. Each Pokemon have a Base and Maximum Level of possible Combat Power, ones that very on the Pokemon Species, its givens Stats, and the Level a Trainer is currently at.
As you increasing your Trainer Level, you will in turn discover more higher-leveled Pokemon in the wild. You will also help increase the possible CP the Pokemon you own can gain; it won't increase their current Combat Points Level, but it will increase the maximum they can reach. Combat Power also play a role in Gym Battles, determining how much damage is inflicted upon the opposing Pokemon. The higher the Combat Power are for either side - along with the Base Power and Effectiveness of Moves - the greater damage that will be done at Pokemon Gyms.
A player can always check on their Pokemon's Combat Power, as well as the option to increase it, by checking out the Pokemon's Profile in their Storage. The Pokemon's current Combat Power will always be displayed above their profile picture, along with a semicircle that shows where they are currently at and the maximum they can current reach (depending on their Trainer's Current Level).
As for increasing a Pokemon's Combat Points - it's actually a pretty simple process. As you catch more and more Pokemon, you will receive more and more Stardust. If you catch the same Pokemon you are trying to power up - as well as by obtaining and using Rare Candies received from defeating Raid Battles - you will also receive their given Pokemon Candy. Both of which - Stardust and Pokemon Candy - are essential and necessary with increasing your Pokemon's CP. You will need at least one of the given Pokemon's Candy, as well as a varied amount of Stardust in order to power up a Pokemon (which increases as your Pokemon's Combat Power increases, starting as low of a cost as 200 Stardust and one Candy per Level Up, going up substantially from there).