Machine-readable is defined by the Government Performance and Results Act (GPRA) as any "format in a standard computer language (not English text) that can be read automatically by a web browser or computer system. (e.g.; xml)."
Machine-readable data is data (or metadata) which is in a format that can be understood by a computer. There are two types: human-readable data that is marked up so that it can also be read by machines (examples; microformats, RDFa) or data file formats intended principally for machines (RDF, XML, JSON).
Publishing public data in an open, standard, machine-readable format is a best practice (good operating practice).