On most entry forms and editors you use a drop-down list is used to make selection. For example, on a sales order you select a customer, ship to address and ship from location with a drop-down list.
You can click the arrow and ‘drop-down’ the list then scroll through it until you find your selection and click it. You can also start to type, for example, the name of the customer, the drop-down list will start to auto-filter then click the TAB key to select it.
On the top of most drop-downs you will see <add new> which allows you to add a new record on the fly.