The definition of an antonym according to Webster’s dictionary is: –noun

a word opposite in meaning to another. Fast is an antonym of slow. Compare synonym (def. 1).

Simply put, they are considered opposites of each other.

One person, or several people may lead, but the others will follow.

I can break something, but if I want to use it later it needs to be fixed.

You can be really swift, but if you do not get your daily exercise in you may become slow.

These are examples of how the words you’ve posted above are dependent on each other, and without one another it would be hard to comprehend.

They are opposites, and need one another in order to function rationally.

High / Low

Light / heavy

fast / slow

and so on. Each is dependent on the other