`timescale in Verilog

Formate : `timescale time_unit base / precision base
Description : Specifies the time units and precision for delays:

• time_unit is the amount of time a delay of 1 represents. The time unit must be 1 10 or 100
• base is the time base for each unit, ranging from seconds to femtoseconds, and must be: s ms us ns ps or fs
• precision and base represent how many decimal points of precision to use relative to the time units.

Example : `timescale 1 ns / 10 ps

Indicates delays are in 1 nanosecond units with 2 decimal points of precision (10 ps is .01 ns).

Note : There is no default timescale in Verilog; delays are simply relative numbers until a timescale directive declares the units and base the numbers represent.

More about Verilog can be found here