Data conversion is the process of transferring/translating data from one format to another. In the context of Open Data SE, typically this involves converting data from a proprietary data format, to an open data format.
Data conversion is ideally an automated process where software is utilized to perform the conversion(s). Data of poor quality can make such processes extremely difficult and even impossible to automate. Open Data standards are reusable agreements making it easier to publish, access, share, and use data; as such they play a critical role in conversions/conversionability.