The entity selector is a special field type you'll see in the Learndot administration which allows for the selection of other entities as a field value (e.g. selecting the Contact teaching an Event through its instructor field).
Selecting an Entity
The entity selector will appear on edit screen whenever a field requires an entity to be selected. To select an entity:
Start typing the name of the entity you wish to select
Left-click on the entity you wish to select in the dropdown list
Clearing the Field
To clear a selected entity, click the small cross in the field as shown in the following screenshot.