The next property, unit or constant matching the search will be displayed, if found. You can continue searching for further occurrences using the Find Next button until no more matches can be found. In the example above the search will find various physical properties of water, but also units containing the term "water", for example inches of water as a measure of pressure.
Some of the properties use scientific names, such as "mass", which are not always the ones in common use. For example someone wishing to convert kitchen measures is more likely to be thinking in terms of "weights" than, say what "mass" of flour is required to bake a cake. If you search for the word "weight" for example, you will find the property "mass" because the database stores the common names for most properties. You can see these common names (or aliases) when you edit units. They are separated by semicolons, and only the first is normally shown.