A Field names a field to be retrieved with a partial response. See

Partial responses can dramatically reduce the amount of data that must be sent to your application. In order to request partial responses, you can specify the full list of fields that your application needs by adding the Fields option to your request.

Field strings use camelCase with leading lower-case characters to identify fields within the response.

For example, if your response has a "NextPageToken" and a slice of "Items" with "Id" fields, you could request just those fields like this:

svc.Events.List().Fields("nextPageToken", "items/id").Do()

or if you were also interested in each Item's "Updated" field, you can combine them like this:

svc.Events.List().Fields("nextPageToken", "items(id,updated)").Do()

More information about field formatting can be found here:

Another way to find field names is through the Google API explorer: