This is the rate at which data is written to or copied to and from a flash drive. It is given in MB/s (megabytes per second). High speeds are most important when writing or reading large volumes of data and the differences in speed with slower flash drives can be felt in the tens of minutes.
Write speeds indicate how quickly informatin will be copied to the flash drive and read speeds are how quickly data can be copied to the connected device. High read and write speeds are always great.