Dever is an Everscale testnet token, which is a native token of the net.ever.dev testnet. It has no value and is given to anyone who wants it.
To get the number of Devers you need, write to the Assistant or use the Telegram bot.
Since the testnet is used to test various aspects of the system, it may occasionally undergo a complete reboot, which erases all account data and transaction history. In this case, you can request Devers again.