In addition to defining security around users and teams, a more minute level regulation of security can be done around a field. This applies to only custom fields. Field security profile is used to give access to fields that have been enabled for field level security to users other than the default System Administrator. Read, Update and Create privileges to these fields are given. The fields enabled for field level security are seen with a small key beside the name indicating that its status is secure. This security permission can be granted to users or teams.
Creating a new field form
Field with Field Security enabled