Once means on one occasion.
- I only went once.
Once can also mean at some time in the past.
- Once (upon a time) there was a princess.
Note that to refer to an indefinite time in the future, we use sometime or one day, not once.
- Come up and see me sometime. (NOT --- see me once.)
As a conjunction
- I would like to go for a walk once the rain has stopped.
- Once he had found somewhere to live he started looking for work.
Search the Dictionary of Practical English Usage