What is safe sleep?

Some Mac computers have a feature called safe sleep. With safe sleep, each time the Mac goes to sleep, the current state of the Mac is saved, including open apps and open documents. Safe sleep prevents your information from being lost if the Mac is completely shut down during sleep, which could occur if the battery runs down or is replaced.

If your Mac needs to access the saved state when it wakes up, a progress bar appears until your Mac is ready to use.