The system is built so that the signal is tested only at particular times, and that changes in the signal occur between these times.
Digital systems are built so that the on/off (binary) value is tested only at certain points in time. This gives the wire (or other device) time to change. This is why computer systems have a clock. The clock generates ticks, which are points in time when signals may be measured. In the picture, T1 and T2 are ticks.
The clock keeps all these time points synchronized. Faster clocks mean wires can be tested more times per second, and the whole system runs faster.
Processor chips are often described in terms of their clock speed. Clock speed is measured in Hertz, where one Hertz is one clock tick per second. The symbol MHz means megahertz, a million (106) clock ticks per second. The symbol GHz means gigahertz, a billion (109) clock ticks per second.
A 2 GHz processor checks binary values two billion times in each second. In between these times values are allowed to change and settle down. The faster a processor chip is, the more times per second values can be tested, and the more decisions per second can be made.
Which is faster, a 2 GHz processor or a 4 GHz processor?