From TalonRO Wiki

How flee is calculated:

Flee is determined by both your base level and your AGI (agility).

Actual Flee = Skill Bonus + 100 + [Level + AGI + (LUK/5) + Item Bonus] * [1 - ((Mobs -2) * 0.1)] 

Item Bonus is bonus granted from Items or Cards, Skill Bonus is bonus directly granted from Skills, and Mobs is the amount of enemies that are attacking you. Buffs ("Agility up" and the like) are factored in as "Skill Bonus". Using the formula no matter how high your Flee is, if 12 enemies are attacking you you will get hit. Flee is also factored in AFTER Perfect Dodge

The formula for Flee Reduction is as follows:

Flee = (flee * (100 - (attacker_count - (battle_config.agi_penalty_count - 1))*battle_config.agi_penalty_num))/100;

Overly simplified, it means after 3 mobs you have a flee reduction and each addition mob reduces flee by around 10%. No matter how high your flee is you will get hit if 12 mobs are attacking you (this does not factor in perfect dodge, since its calculated before Flee)

Skill bonuses include "Improve Dodge" for Thief Class and Super Novice, and "Flee" for monks. The best of the flee cards are listed at the bottom.

What is it good for?
Flee is the answer to low VIT. Characters with High VIT typically don't worry about flee and instead work on damage mitigation or healing/recovery. Characters with high flee typically don't worry about high VIT, don't need a large amount of HP if you aren't getting hit. Flee cannot dodge most magic and/or skill attacks.

There are too many flee cards and gears to list here (since AGI has a direct impact on flee) so here are a few of the more impressive modifiers:

Card.gifKavach Icarus Card (+20 flee if refine 0~4)
Card.gifMysteltainn Card (+20 flee)
Card.gifNine Tail Card (+20 flee if refine 9+)
Card.gifStormy Knight Card (+20 flee)
Valkyrie helm.pngValkyrie Helm[1] (+20 AGI and slotted)
Card.gifWanderer Card (+20 flee thief class only)