What does the root word most mean?

The root word "most" doesn't have a specific meaning on its own. It's actually a superlative suffix that means "to the highest degree" or "in the greatest amount."

It's added to adjectives and adverbs to form their superlative forms, like:

* big → biggest

* fast → fastest

* much → most

* often → most often

So, "most" itself doesn't have a meaning independent of the word it's attached to.