The most important Interaction is the Attack Interaction, which is used whenever a weapon, body part, or missile would come into contact with a target. The Attacker first makes an Attack Roll (1d20) against the target's armor class (AC). If the attacker is proficient with the ability they are using, they add their proficiency bonus to the roll. If this roll is equal to or greater than the target's AC (typically 10 + AV + P if the target is proficient in their armor), the attack Hits. If the roll is less than the target's AC, the attack Misses. A natural 20 on the die is always a Hit, and a natural 1 is always a Miss. The consequence of a Hit is described in the "On hit" section of the ability that's used.
If the Hit is from a natural 20, then the attack is a Critical Hit and the effect of the ability "goes Critical". If this changes anything about the effect, then it's described in the ability's "On critical hit" section.
Any creature that is up may choose to be hit by an attack, even if the attack would normally Miss. This is called "Taking the Hit". They cannot choose to take a Critical Hit.