An IPotion object refers a potion in the game.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import.
import crafttweaker.potions.IPotion;

You can get such an object through the use of the Potion Bracket handler

ZenScript
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<potion:minecraft:strength>;

Zengetter	What does it do	Return Type
Zengetter nome	What does it do Returns the potion's internal name	Return Type string
Zengetter badEffect	What does it do Returns if the potion effect is bad	Return Type bool
Zengetter liquidColor	What does it do Returns the potion's color	Return Type int
Zengetter liquidColour	What does it do Returns the potion's colour	Return Type int
Zengetter curativeItems	What does it do Returns a list of all curative Items for the potion	Return Type List<IItemStack>
Zengetter hasStatusIcon	What does it do Returns whether the potion has a status icon	Return Type bool
Zengetter isBeneficial	What does it do Returns whether the potion is beneficial	Return Type bool
Zengetter isInstant	What does it do Returns whether the potion is instant	Return Type bool

An IPotionEffect is a Potion that has a duration and amplifier. You can use these two methods to get one from an IPotion:
As one might guess, both return an IPotionEffect object.

ZenScript
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pot.makePotionEffect(int duration, int amplifier);
pot.makePotionEffect(int duration, int amplifier, boolean ambientEffect, boolean particlesShown);