Surprising is a chance event that can happen to the enemy or the hero party whenever combat happens. The chance of this happen can be influenced by the light level but does not ensure the enemy being surprised or preventing the party from being surprised. Overall this can make even a tough encounter end quickly or make even a simple one a challenge.
When the hero party travels through dungeons there is a chance of surprising (or being surprised by) their enemies at the beginning of combat. Surprising the enemy can easily give the party a massive edge since it gives a turn to land attacks, buff or debuff the enemies. However in turn being surprised can horribly affect the party since it shuffles up the hero line which can easily get a group wiped out or badly injured.
When your Light Meter is between 100 (Radiant Light) and 50 (Dim Light) you have a higher chance to surprise the enemy groups. However the chance is still random and even in high light the hero party can still become surprised by the enemy.
Enemies:When enemies are surprised a orange (!) will appear over their head with an arch similar to the stun effect. All enemies will take no action during this turn, giving the heroes a chance to possibly wipe out any monsters. However attack can still be dodged so don't expect a quick win. After one turn the enemies will not be surprised and fight normally. Enemies cannot be surprised again during combat or if the party recently fled from them.
The Heroes: When the hero party is surprised all the characters will quickly have a orange (!) appear over their head before the group is randomly shuffled. This will most likely mess up the combat line which can have front line fighters like the Crusader and Hellion end up in the very back while your Plague Doctor and Occultist are up front.
This will force you to move your heroes around to properly fight since some or all of a character's abilities may become unusable because of position. Either end combat quick, use quick moving heroes like the Grave Robber and Jester or certain movement abilities to more quickly fix the hero line.
The one upside is that character speed works the same like normal combat so the only disadvantage is having the party line shifted. It is useful to have your heroes have a versitile skill set to be effective outside of their normal position.