Can you explain what is meant by data migration or changes in database structure?

The process of moving data or changing the structure of a database is usually called migration. It involves a set of operations that modify the database schema — adding or removing tables, changing columns, indexes, etc., as well as transferring or transforming data.

In the context of Node.js, migrations are often managed using special tools (such as Sequelize, Knex), which allow describing changes in the form of scripts and applying them sequentially.

Example of a migration using Knex:

exports.up = function(knex) {
  return knex.schema.createTable('users', function(table) {
    table.increments('id').primary();
    table.string('name');
    table.string('email').unique();
  });
};

exports.down = function(knex) {
  return knex.schema.dropTable('users');
};

Thus, migrations help manage database changes in a controlled and repeatable manner.