Based on the Whitepaper found here, logical time is described as:
For this purpose, the creation of an outbound message is considered an atomic event, logically dependent on the previous message created by the same transaction, as well as on the previous transaction of the same account, on the inbound message processed by the same transaction, and on all events contained in the blocks referred to by hashes contained in the block with the same transaction.
As a consequence, outbound messages created by the same smart contract have strictly increasing logical creation times.
In essence, Logical Time is the method by which validators sort transactions and messages logically. Although the key concept is that a higher value of Logical Time will be executed later by the validator.