RDD does not fit in memory

if the RDD does not fit in memory, some partitions will not be cached and will be recomputed on the fly each time they’re needed.
What is the meaning of fly and recomputed

Did not understand ;; suppose I have 10 gb and 9 gb fit

Then remaining 1 gb will be fit on recomputation Each fly

Please explain this

In simple words

why can’t we use MEMORY_AND_DISK in this scenario