The idea of one to many (1:M) concerns the joining of rows to each other, specifically cases where a single row in one table corresponds to many rows in another.
1:M is one-directional, that is, any time you query a 1:M relationship, you can use the 'one' row to select 'many' rows in another table, but you cannot use a single 'many' row to select more than a single 'one' row.
For most cases, working with a 1:M relationship requires us to understand Primary Keys and Foreign Keys.
A Primary key is a column in a table where any single row of that column represents a single entity, or, selecting a value in a primary key column results in exactly one row. Using the above examples, an EMP_ID represents a single employee. If you query for any single EMP_ID, you will see a single row representing the corresponding employee.
A Foreign Key is a column in a table that corresponds to the primary key of another different table. From our example above, the MGR_ID in the EMPLOYEES table is a foreign key. Generally to join two tables, you'll join them based on the primary key of one table and the foreign key in another.