Cache is actually a good thing. While burning a CD or DVD, the writer needs a steady stream of data from the hard disk / other sources for the data to be burnt successfully. To ensure this, most burning programs "cache" or buffer the data in the RAM (which is significantly faster than the hard disk) so that it can be sent to the drive in a steady stream. In a computer, the hard disk may get busy doing other stuff and is not able to send data in a constant stream to the burning app.
Caching ensures that the data is read ahead and stored in the RAM so that the burner never goes hungry and results in a failure. A second level of caching or buffering happens in the drive (most modern drives have around 2 MB of buffer). In Nero, leave the options in their default settings. Don't disable the cache or modify the settings unless you know what you are doing. HTH.