There are four different stats troops have: attack, armor, life, and magic.
Attack is the base amount of damage that a troop deals on a skull hit (if it is at the front of a team). It can be temporarily nullified by the Entangle status and increased or decreased by a wide variety of troops. Matching a set of 4 or 5 skulls increases the damage of a skull hit by 1 or 2 damage, respectively. There are also some abilities that scale from a troop's attack.
Armor and life (also called health) are very similar in that they make up a troop's defenses - if a troop has 8 armor and 12 life, it needs to be dealt 20 damage for it to die. Normally, damage is dealt first to armor, and then to life if no armor remains. The exception is for true damage, which ignores armor completely. The major difference between the two stats is that a troop can survive without armor, but will die if its life is depleted (even if it has some armor left). There are multiple troops who can instantly remove all armor from another troop (since this is non-fatal). There are troops whose abilities scale with armor or life.
Magic is the basis of a troop's ability strength. A typical ability will be measured similar to 'deals 3 + magic damage', so if the troop has 12 magic, that ability deals 15 damage. Magic can be temporarily nullified by the Web status. The ability to increase or decrease troops' magic is fairly rare due to the implications, but there are troops who can do it. Some troops have abilities that don't scale with magic on a 1:1 ratio.
The central pane shows armor, life and attack; the left pane shows magic (because it pertains to the troop's ability).
In the picture above, the Lance Knight has a magic stat of 9, reflected in its Joust ability's armor gain of 10. Joust has a gain of 1 + Magic, meaning if the Lance Knight were to have 5 magic, it would gain 6 armor when cast.