DPS, or damage per second, is damage your character can do every second.

Wonder how that number got there?

Well, there may be different ways but a simple way to find it out (even tho it's already figured out for you) is to:

1. Find the average of your minimum and maximum damage
Example. 20(min) + 38(max) = 58/2 = 29(average)

2. Then find the speed of the weapon, which can be found by reading the item stats in your inventory.
Example. 1.3 Speed

3. Take the average damage and divide it by the speed.
Example. 29(average) / 1.3(speed) = about 22(Dps)

And there you have it