Drupal, by default, differentiates between base fields (which are basically hard-coded and always belong to that entity), and "custom" fields which are added to an entity via the UI and can be different per entity bundle (e.g. they can be different per content-type). In order to prevent conflicts, Drupal adds the prefix "field_" to all custom fields. This, however, makes it very tedious to access those fields, because you are usually working with those custom fields much more frequently than with the few ones that are built-in - and you have to remember prefixing those name with "field_" each and every time.
Now, DrupalX makes the "field_" prefix completely optional for most cases. You can still use it if you actually have a conflict (i.e. a name that exists as a base field as well as a custom field). However, we'd suggest you try to avoid creating conflicts in the first place and just choose names for your fields that are not normally used for base fields (e.g. "title", "created" etc.) anyway.