From Minter Wiki

Stake is the amount you delegate to a Validator's slot. Validator's stake is the sum of delegated stakes from all delegators.

Minimum stake

It is worth to remember that the number of validator's slots cannot exceed 1000. When all slots will be occupied, validator will start to limit the minimum stake amount. That means if validator's minimum stake is X BIP, one must send a transaction with a stake bigger than X BIP for a successful delegation. If another user sends a transaction, which is larger than yours to the same validator, your stake will be automatically "knocked out" from the slot and will be returned immediately to your wallet.

See also