Concurrency is when many users are interacting with a system or many changes are being made at once.
A DBMS needs to use concurrency. However, the database needs to prevent two people using the data in a conflicting way. For example, two people visiting an online shop might try to buy the final product in store. The database would need to make sure only one person could buy it.
A change in a database is called a transaction. Changes to databases must conform to ACID rules: