Every instruction needs some timing signals for its execution. The exact number of timing signal will depend upon how long it takes for an instruction to execute completely for example-: an instruction with a subroutine or a system call will require more timing signals than an ordinary instruction.
Subroutine here, is another instruction that has to be invoked in order to complete the parental instructions in computer definitions timing signals are used as conditions for example-:
A timing diagram is nothing but a plot of at which time the respected instruction will trigger the plot is basically made of vertical lines each lines representing significant time .the instruction are plotted by horizontal lines the plot of an instruction takes a rise on the appropriate time of trigger similarly takes a fall at the end of that time
The rise and fall may be depend on some other conditions as well.