Effects are buffs and de-buffs, which we call 'buff' and 'down' respectively, to your unit's statistics. Effects can completely change the outcome of combat.
For Example: I have HP: 30 and DF: 0, you have AP: 8, AC: 1, and AT: 4, you expect to do 32 Dmg and kill me, I play the Card 'Aponia' (+4 DF) and you go from doing 32 Dmg and killing me to doing 0 Dmg.
You will know if a unit has Effect(s) on it because you will see icon(s) for those Effects on the unit's Stat Readout. If you scroll over the icon in the Stat Readout you might see something like this:
at buff ------------------------- +2 (0) +3 (1) +4 (∞)
What does the above mean? Effects are displayed in the following format:
effect_name ------------------------- amount (duration)
Which means that the unit in our example currently has 3 different AT buffs:
+2 AT (until its next Upkeep) +3 AT (until its 2nd Upkeep) +4 AT (indefinitely)
Cumulatively, this unit has a total of +9 AT added to its Base AT.
The durations of Effects are decremented and applied at Upkeep. When this unit gets its next Upkeep, its AT buff effects will read as follows:
at buff ------- +3 (0) +4 (∞)
The +2 (0) has been removed and the duration of the +3 (1) was decremented down +3 (0).
The Card 'Eris' removes all beneficial effects from a unit. If you were to play 'Eris' on this unit, then both of the AT buffs would immediately be removed from the unit and the unit's AT would return to its Base AT. If you did not play a card like 'Eris' on this unit, the +3(0) would run out on the unit's next upkeep, but it would keep the +4 (∞) buff forever.
Effects on AT, DF, and CC work as described above (the effect modifies the Base Value so that you get a new Current Value for the natural duration of the Effect, or until the Effect is removed with a card like 'Eris'). Effects on HP and AP work a bit differently.
The Card 'Mutagen' applies 4 different Effects to a unit. 'Mutagen' does '+1 DF and +2 AT indefinitely. -2 AP and +2 HP on each of target's upkeeps. As soon as you play 'Mutagen,' the unit's Current DF Value gets +1 and its Current AT Value gets +2, this lasts for the duration of the effect. On the other hand, the AP and HP Effects have no immediate impact on the unit, the unit's AP and HP are not immediately affected. Not until the unit's next upkeep will the AP and HP Effects have an impact. At all of the unit's future upkeeps, the unit will have its Current AP refreshed to its Base AP, and then it will get -2 AP to its Current AP. The unit will then also get +2 HP to its current HP. If this unit then has a card like 'Ground State' played on it ('Ground State' removes all effects from a unit), the unit's Current DF and AT Values will return back to their Base Values, but, the unit's Current HP and Current AP will not be changed. So, if it is your Movement Phase, you have a unit with these stats:
AT: 4 AP: 6/6 DF: 2 HP: 30/30 AC: 1
and you play 'Mutagen' on one of your units, their stats will immediately look like this:
AT: 6/4 AP: 6/6 DF: 3/2 HP: 30/30 AC: 1
Notice that your AP and HP have not changed, yet, they won't change until your next upkeep. So, if you now attack a target, you get the benefit of the +2 AT and you can still take 6 shots. After you attack, your stats will look like this:
AT: 6/4 AP: 0/6 DF: 3/2 HP: 30/30 AC: 1
You used up all your AP attacking, so your Current AP is down to 0. You end your turn. Your opponent takes their turn and then they pass the turn back to you and you get an Upkeep, now your stats look like this:
AT: 6/4 AP: 4/6 DF: 3/2 HP: 32/30 AC: 1
Your AP refreshed back to its Base AP of 6, and then you got -2 AP from the effect applied by the 'Mutagen' Card . You also gained +2 HP. If you were to now play a Card like 'Ground State', your stats would read as follows:
AT: 4 AP: 4/6 DF: 2 HP: 32/30 AC: 1
Notice you are now back to your Base AT and DF values and your HP and AP are unchanged. Any HP you gained in the past from effects are not removed when the effects are removed, so you get to keep the 2 HP. With AP as well, removing the effect does not immediately restore your AP to its base value, so you still have only 4 AP for this turn. Now that the effect was removed, on your next Upkeep your unit will refresh back to its Base AP and the unit's stats will look like this:
AT: 4 AP: 6/6 DF: 2 HP: 32/30 AC: 1