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 only tested at certain times so the electronics can change its state between these times. This is why computer systems have "clocks" — to keep all these times synchronized. Faster clocks mean wires can be tested more times per second, and the whole system runs faster.
Processor chips (and the computers that contain them) 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 mega Hertz, a million clock ticks per second. The symbol GHz means giga Hertz, a billion clock ticks per second.
A 700 MHz processor checks binary values 700 million 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?