Earned run average is one of those stats where the lower it is, the better the pitcher. A pitcher’s ERA is calculated by the number of earned runs they’ve allowed (ER), divided by the number of innings pitched (IP) multiplied by 9 (the traditional inning length of a game).
A pitcher who pitched 150 innings and allowed 100 earned runs would have an ERA of 6.00
Walks plus hits per inning pitched is precisely that, a calculate of the number of baserunners allowed by a pitcher per inning of work. The lower a WHIP, the fewer baserunners allowed by a pitcher. WHIP is calculated by adding hits and walks and dividing it by innings pitched. A WHIP under 1.000 is considered exceptional for a season.
During a seven inning start, say a pitcher allowed 5 hits and 3 walks. Their WHIP would be 1.14 on the game (8 hits and walks/7 innings pitched).
These stats are used to show a pitcher's average amount of Walks or Strikeouts, if they were to pitch nine innings.
If a pitcher had 103 strikeouts across 150 innings, their K/9 would be 6.2. If the same pitcher also gave up 47 walks, their BB/9 would be 2.8.
K/BB is used to show how many more strikeouts than walks a pitcher tosses. The higher the number, the better the ratio. This stat can be used to evaluate a pitcher and get an idea of their control, a pitcher with a lower K/BB tends to have worse control when compared to a pitcher with a higher K/BB.
Taking the pitcher from K/9; 103 strikeouts and 47 walks equals a K/BB of 2.19