The clock tree delivers a clock to all flops in a domain with sufficiently low skew to avoid shoot-thru. This is achieved by balancing wire lengths between the drivers.
The clock frequency is a multiple of the external reference which is commonly sourced from the piezo-effect of sound waves in a thin slice of quartz crystal.
Later on, under power management, we will talk about having a programmable clock frequency, so it's worth noting that the multiplication factor of 10 illustrated in the slide can be variable and programmed in some systems (e.g. laptops).