This is the 1150th most frequent Japanese word.
効い
"効い" is not a standalone word. It is likely part of the verb "効く" (kiku), meaning "to be effective" or "to work (well)."
Here, '効い' (kī) is used to describe the effectiveness or functionality of the medicine.
この薬はよく効いています。
This medicine is working effectively.
In this context, '効い' (kī) refers to whether an advice produces a desired effect.
彼のアドバイスは私には全く効きませんでした。
His advice had no effect on me at all.
'効い' (kī) here is used to indicate that the air conditioning system is functioning properly, resulting in a comfortable environment.