CP is a general indicator of your pokemon's current combat stats.
because stats go up with a pokemon's level, CP is generally higher on higher level pokemon. A pokemon's level can be increased using stardust and candies but cannot go higher than your trainer level + 1.5 (each upgrade is half a level
)
You can roughly see the pokemon's level based on the white arc that appears above it on it's info page. The left of the arc represents level 1 and the far right is your trainer level + 1.5, with how far the arc is filled showing the pokemon's current actual level relative to these :3
While each pokemon of a species share the same base stats (e.g. all pidgies have the same base stats as each other) individual pokemon also have something called 'IVs' which are bonuses unique to them to each of their 3 stats that can range anywhere from 0 to 15 so you'll often find that even pokemon of the same species and level will have different CP values. You can use
IV calculators to try and work out your pokemon's IVs if you like but to be honest it's more important to make sure your pokemon have a good moveset than worry too much about this IV stuff :3