SalesDesktop detects a collision when a certain item has been changed on both your computer and Salesforce since the last synchronization session and these changes differ from each other.
You will be notified about this collision with a pop-up notification window and an exclamation mark over the taskbar icon.

There're several ways a collision can be resolved:
Alternatively, you might want to skip the collision and resolve it later by opening respective dialog window from the SalesDesktop context menu.
Coming up next: Duplicates, duplicates, duplicates…