Metadata is literally ‘data about data’. In the context of weather records, it is a description of the site and its surroundings, the instruments in use and any changes over time, information about observational databases and units used, where the site’s records are archived, and any other details about the measurements that may be relevant.
Why is it important? Because it provides the essential information for any other user of the records to understand more about the location and characteristics of the data, and therefore enables more informed use of the data. For example, your metadata could make it clear that the anemometer in use was at a different height for the first few years of the record, and that records before and after the change are not homogeneous. Such details may be known to the observer but may not be immediately obvious from the records themselves. Metadata are especially important for elements which are particularly sensitive to exposure, such as precipitation, wind and temperature. A comprehensive site and instruments description also allows you and other observers to compare records with a degree of confidence, and to be sure that you are ‘comparing apples with apples’.