Here's a scenario: You say to your partner, "Today, I'm going to scrub the floors, wash the bedding, and clean the bathroom. Oh, who am I kidding. I'm just going to surf the Web."
In other words, "who am I kidding" means: who am I joking?, who am I fooling? who am I pretending to?. The "who" should technically be "whom".
It is difficult to understand, ggonak. Keep asking questions if you don't get it.