Field-level help lets you provide help text detailing the purpose and function of any standard or custom field. You can define custom help text for your organization’s fields to provide users with a helpful description for any field on all detail and edit pages where that field displays.
Follow the below steps to add help text to a field in lightning,
Step 1 : Go to Setup
Step 2 : Select Object Manager and Select the object from the list
Step 3 : Select Fields and Relationship
Step 4 : Select the field you want to add/change help text
Step 5 : Click Edit and Enter the description in Help Text
Step 6 : Click Save
This text displays on detail and edit pages when users hover over the Info icon next to this field.