Metadata is the information (data) associated with a dataset. Thus, it is the description and ancillary information related to a particular experimental measurement. For instance, an x-ray scattering detector image is the data, while the sample name, measurement temperature, experimental x-ray wavelength, etc. would be part of the associated metadata.
Datasets are being coupled with increasing amounts of metadata. This is an opportunity (in terms of more sophisticated automated analysis, data mining, etc.), but is also a challenge in terms of data management.