Meaning of “one” - Learner’s Dictionary

one

pronoun uk us /wʌn/
PARTICULAR PERSON/THING

A2 used to refer to a particular person or thing in a group that has already been talked about:

I've just made some biscuits, do you want one?
Throw those gloves away and get some new ones.
Chris is the one with glasses.
GENERAL formal

any person in general:

One ought to respect one's parents.
one at a time

B2 separately:

Eat them one at a time.
one by one

B2 separately, with one thing happening after another:

One by one the old buildings have been demolished.
one another

B1 each other:

How can they reach an agreement if they won't talk to one another?
(all) in one

combined into a single thing:

It's a vacation and art course all in one.

(Definition of “one pronoun” from the Cambridge Learner’s Dictionary © Cambridge University Press)