Statistics are a representation of how your monster is growing in the games. The stats are:
Accuracy, also called Skill (SKL)
By doing training and drills, a monster's stats will increase. In some games, stats can be increased after winning a tournament as well.
Generally, stats are capped at 999, although there are exceptions. For instance, in Monster Rancher DS, Overdrive monsters may raise stats up to 1500; however, the total of such a monster's stats must be less than or equal to 5994, the total that it would have if all stats were 999.
Some monsters are easier to train at certain stats than others. For instance, Pixies tend to have high intelligence but low defense and life, whereas Golems tend to have high power and defense but low intelligence and speed. These different stat distributions suggest different strategies for monsters; for example, a Pixie might make a good "glass cannon", able to quickly deal an immense amount of damage with its attacks, but unable to survive more than one or two counterattacks (if that). In contrast, a Golem might be a good "tank", taking multiple strikes from a quicker foe without having its hit point percentage drop too low, but knocking down an opponent's life down enough with a single successful attack to win through a knock out. These are only two strategies out of many, of course.