All playable characters and enemies you encounter have stats, numeric representations of their physical and mental combat capabilities. Some of these stats are easily seen by going to the Status menu, while others are partially or totally invisible to the player.
Visible Stats
Health/HP - A character's ability to sustain damage without fainting or dying. You can only continue fighting until this runs out.
Mana/MP - A reservoir of magical energy. Characters can only cast spells so long as you have the MP to fuel it.
Tech/TP - Technical skill, emotional energy, and opportunity all rolled up into one. TP is used to fuel your Special skills.
Attack/ATK - Raw brute strength. Standard attacks and some skills deal more damage if your ATK is increased.
Defence/DEF - Physical endurance. Characters with high DEF will sustain less damage from standard attacks and some skills.
Magic Attack/MAT - Magical proficiency. Most skills are more effective if your MAT is increased.
Magic Defence/MDF - Mental resiliency. Characters with high MDF sustain less damage when targeted by most skills.
Agility/AGI - Grace and speed. Having a high AGI score allows you to act earlier in battle more often.
Luck/LUK - This nearly-useless stat very slightly impacts a character's chance of inflicting or avoiding status effects.
Invisible Stats
Evasion/EVA - The ability to avoid attacks entirely is tied directly to a character's evasion. All player characters and most enemies have 5% evasion by default. However, you can raise or lower your own evasion by equipping certain items, and some enemies have heightened evasion.
Resistance/Weakness - All damage has an elemental type, and some enemies are resilient to or weak against specific elements. Character dialogue and visual clues sometimes inform the player as to which elements are worth using and avoiding, but in other cases you must simply discover these traits through trial and error (or just look up the enemy here).