It's important to rapidly cool your wort for a few reasons.

1) Once the beer falls below the pasteurization temperatures (170-180F), it is susceptible to contamination. Cooling it quickly reduces the risk of contamination.

2) Cooling quickly stops hop acid isomerization if you use whirlpool hops, keeping your beer from getting too bitter and/or destroying delicate hop oil aromas.

We recommend using a counter flow chiller between your boil kettle and primary fermentation vessel. This is the quickest way to chill your wort and keep it from getting contaminated. Another common and less expensive option is an immersion chiller however these are harder to clean and sanitize.