A temporary phone number is a real mobile number you use for a short time to receive a one-time SMS verification code. You choose the service and the country, the number is assigned to you, and the code appears on your dashboard. When the number expires it is released, so it is meant for signups and tests, not for keeping.
If no code arrives before the number expires, your credits are refunded automatically. Delivery depends on the service sending the code, so a code is never guaranteed, which is why the refund exists.
Apps usually check the country prefix, so pick the country the app expects.
Each country page explains the number format, the apps people commonly verify there, and the accounts you should keep on your own number.
Check pricing before you start. Price depends on the service and country.
A temporary number cannot receive a code a month later, so it is the wrong tool for accounts you will need to recover by SMS. Keep banking, payments, health, tax and government accounts on a number you own. Some apps also refuse numbers they classify as VoIP, or limit how often one number can be used; if one is refused, request a new number for the exact service.