Fire Rate, also known as Bullets per second, is a stat that determines the amount of ammo that will be fired by the weapon per second. Several heroes have abilities that can increase their fire rates to shoot faster, but it can also be decreased, primarily through other enemy's items.
The inverse of this is called gun cycle time, which instead measures seconds per bullet.
Golden Statues can provide a permanent increase: +1.5% Fire Rate before 10 minutes, +2% Fire Rate before 30 minutes, or +2.5% Fire Rate after 30 minutes.
Power up runes can provide a temporary Gun buff that provides +20% Fire Rate for 160 seconds.