please explain in erpnext .Please explain the logic behind this.

So for this file ./apps/frappe/frappe/patches/v6_16/

you wish to know what purpose the field serves or instead why it was renamed?

    for table in frappe.db.get_tables():
            columns = [r[0] for r in frappe.db.sql("DESC `{0}`".format(table))]
            if "_starred_by" in columns and '_liked_by' not in columns:
                    frappe.db.sql_ddl("""alter table `{0}` change `_starred_by` `_liked_by` Text """.format(table))

yes exactly for what purpose the field serves. Please explain.

It is for the same reason Discuss (here) and many other platforms have the little heart icon. For validation and appreciation.

Here is the thinking behind it: