What is Track Field History in Salesforce?

While creating the object, we can enable ‘Track Field History’.
If, we enable ‘Track Field History’ user can see ‘Set History Tracking’ button under ‘Custom Fields & Relationships’ section which is available on the object detail page.
By clicking on ‘Set History Tracking’ button, we can enable tracking for the fields whichever we want.
To see the object history, go to object’s record layout and add Object Name History related list to the layout.
Whenever a user changes field value from one value to another value, it will show the history of the field in ‘Object History’ related list.

In Track Field History, you can select certain fields to track and display the field history in the History related list of an object. The field history data is retained for up to 18 months.