I can't understand the meaning of 背が高い人. From what I've heard you can't just say 高い on a non living object, so what is the meaning of 背 exactly?
高い has very wide meanings including high, tall, expensive, hot, and so on. but usually you have to add 背 referring tall. 背 means tallness of human beings. 背が高い人 translates in verbatim "a person who is high in tallness".
背が高い人 means "tall person".
高い人 (without 背) has a different meaning.
高い人 means "arrogant person", someone who looks down on other people.