Individual value (IV) is hidden stats of Pokemon. It is associated with the highest possible CP and HP. Each individual Pokemon can raise its CP and HP higher depending on how much individual value it has. It is decided when you get Pokemon: capturing or hatching eggs.
Full IV for HP and CP
In Pokemon Go, IV ranges from 0 to 15 for HP and from 0 to 30 for CP. Based on IV, same kind of Pokemon can have different HP and CP. For example There is 9 percent difference for max HP and 15 percent difference for max CP between two Dragonites: the one with the highest IV for each stats and the another with the lowest IV for each stas.
Range of IV
Example) The highest possible CP and HP of 2 Dragonites with different IV
Dragonite A: 15 IV for HP and 30 IV for CP
Dragonite B: 0 IV for HP and 0 IV for CP.
Should I select Pokemon to power up based on IV?
No. You don't have to. Here are the reasons why Pokemon Go players do not have to consider IV.
Increasing trainer level is the most important.
To see the highest possible value of CP, you have to increase your trainer level. To reach level 40, you need to gain 2,000,0000XP. That is very hard work. Once you trainer level is at 40, you may look for the most powerful one based on IV.
Large amount of candies is needed.
As well as trainer level, you need to get a lot of candies to raise CP of Pokemon to see their IV.
Moves are more important
Relative to IV, moves are more important for winning gym battles and easier for Pokemon Go players to see.