Gearing on bikes is just like gearing in cars. Lower gears = better acceleration, more RPM, lower top speed; higher gears = poorer acceleration, fewer RPM, higher top speed.

On a fixed gear bike, there are 2 gears which determine your gear ratio (as above).

Your rear wheel has a cog (held on with a lockring). Your cranks have a chainring (bolted to the cranks, typically).

For riding in the streets, you want a middling gear ratio. Something low enough that you can climb hills without mashing but high enough that you can speed through traffic without spinning out. An example of that would be something like 46:16. That is, you would have a chainring with a 46 tooth count, and a cog with a 16 tooth count. These ratios can be expressed as gear inches (different ratios can still yield the same performance). 46:16 will give you 77 gear inches which is normal for the streets.