Safes have a so-called nonce. This is a number assigned to a transaction to ensure that each transaction can only be executed once. Every time a new transaction is created, the nonce is incremented by +1 and assigned to the transaction, essentially determining the position in the queue of transactions for a given Safe.

Only transactions that have a nonce of last executed transaction +1 can actually be executed at any given time. Transactions with a higher nonce are staged for execution in the queue. So whenever a transaction gets executed, the next transaction in the queue will be made available for execution, given it has collected enough signatures.