When you open the agenda of your colleague, you most likely see their planned calls, tasks and meetings, but sometimes you might encounter something as shown below: a grey block.
This item belongs to the private agenda your colleague has synced with Teamleader. The idea is that you and your colleagues can plan everything work-related in Teamleader, while keeping everything personal in your private agenda in Google, Exchange or iCloud. By syncing both accounts, Teamleader pushes all planned items from Teamleader to your integrated agenda and everything from your integrated agenda back to Teamleader. Those items will be displayed as a grey block instead of showing all private information. This helps you determine whether your colleague is busy or not, be it with private matters or work.
However, your colleague does have the option to convert this calendar entry into a visible item by double clicking on it and converting it into a meeting or task.