These are units for measuring data use – like cm, metres and kilometres are for distance.
KB (kilobyte): 1,024 KB = 1MB.
MB (megabyte): 1,024 MB = 1GB
Vodafone’s mobile plans talk about GB of NZ data, which is data you can use in New Zealand to upload (i.e. pictures to Facebook) or download (streaming a video) stuff on the Internet.