When a mirroring system is using synchronous mode, it can be in one of two states: synchronizing or synchronized.
Once an operational server starts and determines that it will act as the mirror, it first requests any log pages from the primary server that it does not already have. This may involve copying pages from log files other than the current active log on the primary server. As it receives these pages, the mirror applies the changes they contain to its copy of the database. Once all pages from the primary have been received, the primary and mirror are in a synchronized state. From that point onward, any changes committed on the primary must be sent to the mirror and acknowledged by the mirror.
In asynchronous and asyncfullpage mode, the mirror requests log pages as above; however, the two servers never enter a synchronized state. Once the mirror has requested all log pages available at the primary, the primary is notified that it must send any updated pages to the mirror.