"Fall off" can be interpreted in few different ways, at least:
- As Katie mentioned, to fall to the ground, or to lose balance typically from higher ground or object. Examples: Tommy fell off his bike. The man fell off the cliff*
- To decrease, or lose value.
Few idioms: To fall off the radar = to disappear ; to fall off the wagon = to start drinking/using drugs again after being sober ; fell off the back of a truck = when speaking about an item, meaning that the item is (likely) stolen goods ; * = fell off a cliff can also mean to become less successful
"Fall" is just a bit more generic and you'll find plenty of good examples of it's usage in dictionaries. https://www.collinsdictionary.com/dictionary/english/fall
I'm not sure if this answers your question, but my advice would be then to just look up different example sentences to understand the difference between the two.