Luckily math is my favorite subject, so I'll put everything in the bulbapedia page into simpler terms:

catch value is a formula used to whenever you catch pokemon, there are several factors taken into account. This is the formula:

CatchValue = ((( 3 * Max HP - 2 * HP ) * (Catch Rate * Ball Modifier ) / (3 * Max HP) ) * Status Modifier

Here is what this all means:

Max Hp-The lower the pokemon's Hp, the more likely you are to catch it, which is why false swipe is an excellent set for this, as it only leaves one HP.

catch rate-the catch rate is a preset number given to every pokemon. it can be anywhere from 0 to 255. the higher it is, the easier it is to catch. this is why Magikarp, with a catch rate of 255, is MUCH easier to catch than a Kyogre, who has a catch rate of 5 (you would only have a 0.7% chance of catching him at full health using a pokeball)

ball modifier-the type of ball you use influences the odds of catching pokemon, here is the list of what every type of pokeball has for its value:

http://www.serebii.net/games/capture.shtml

basically, an ultra ball works better than a pokeball.

status modifier-If the pokemon you're trying to catch has a status on it, it's more likely to be caught. here is the table for what number you put in for status modifier:

Status Figure

Frozen 2

Sleep 2

Paralysis 1.5

Burn 1.5

Poison 1.5

None 1

after all of this, whatever you get for the catch value is put into this equation:

Catch = 1048560 / √(√(16711680 / CatchValue)) = (220 - 24) / √(√((224 - 216) / CatchValue))

the catch value will be put through this equation, and every time the pokeball shakes, a random number is selected, if your catch is higher than this pokemon, then it will shake again, and then one more time to determine if the pokemon is caught. If your catch is lower, then the pokemon won't be caught. Sorry about the length of this answer, I hope I put it into simple terms for you!