The lifeline is just a representation of CPU activity. Each box represents some processing that has to be performed by the CPU for the peripheral to operate.
In between the boxes the peripheral will run on its own, and the CPU is free to go into sleep (system ON), or to do other tasks.
The length of the boxes is not exact, it is just a coarse representation of how much processing each step involves, but the number of processing steps (boxes) is usually accurate.
