The time (for something to be done) is passing quickly; hurry up. For example, The clock is ticking on that project. This allusion to a stopwatch is often used as an admonition to speed something up.

Does a clock tick or click?

is that tick is to make a clicking noise similar to the movement of the hands in an analog clock or tick can be to go on trust, or credit while click is to cause to make a click; to operate (a switch, etc) so that it makes a click.

How much time is a clock tick?

A clock tick could be 1 millisecond or microsecond while the clock cycle could be 0.3 nanoseconds. On POSIX systems CLOCKS_PER_SEC must be defined as 1000000 (1 million).