Radioactive isotopes have a very wide range of half-life values. Some have a half-life of a tiny fraction of a second, while others have a half-life of billions of years. A half-life of 4.5 × 109 years is written in standard form, which makes very large numbers easier to handle.
The hazard from a radioactive material depends on its half-life. An isotope with a short half-life decays quickly, so for the same amount of material it gives out a lot of radiation at first. It is very dangerous at the start, but the radioactivity soon dies away, so it does not stay a problem for long.
An isotope with a long half-life decays slowly. It gives out radiation at a lower rate, but it stays radioactive for thousands or millions of years. It is a hazard for a very long time, so it must be stored safely for that whole period.
This is why the half-life matters when a radioactive material is chosen, handled or stored.