Prefixes are used to simplify the writing of very big or very small numbers.
For instance, instead of having to write 12 500 000 m every time, you can simplify it by using the prefix mega (M), hence it can also be written as 12.5 Mm. Similarly, 0.00054 s can be written as 0.54 ms.
If you are unsure or confused with the unit conversions, you can refer to the following videos.
1) Prefixes can be used for different physical quantities units.
2) Unit conversion with examples
3) Unit conversion for units of area and volume
4) Unit conversion for units of speed and density