It returns the first 50 values of a field named "field". You might want to check out the follow page for more information about retrieving data. It focuses mostly on hypercubes, but list objects are accessed in a similar way.
For large data sets, you rarely want to download the entire set in one go. Access to data from the engine is therefore done by accessing only selected parts (pages) of the data at a time. You can find more information here: