What's the meaning of "What the hell." as a answer?
The phrase is used in the below situation in american TV "How I met your mother".

One day, Ted, the hero of the drama, saw a beautiful lady in a bar and he asked her a dinner.
Ted : You wanna go out with me Saturday night?
She : Sorry, I can't. I'm going to Orland for a week on Friday.
Ted : I think this is a long shot, but how about tomorrow night?
She : Yeah, what the hell.

I understood the answer meant a Yes from the image, but I'd like to know the detail of this answer.

Question

What's the meaning of "What the hell." as a answer?
The phrase is used in the below situation in american TV "How I met your mother".

One day, Ted, the hero of the drama, saw a beautiful lady in a bar and he asked her a dinner.
Ted : You wanna go out with me Saturday night?
She : Sorry, I can't. I'm going to Orland for a week on Friday.
Ted : I think this is a long shot, but how about tomorrow night?
She : Yeah, what the hell.

I understood the answer meant a Yes from the image, but I'd like to know the detail of this answer.

Answer

Here, it means something like 'why not?' or 'what do I have to lose?'

It is a low-key affirmative response. I remember this episode. Robin was excited that Ted had asked her out, but she didn't want to show him that. Hence her response. She wanted to 'play it cool' and act like it was no big deal although she was excited.

Answer

In this situation, 'What the hell' means 'I know it's a bad idea but - what the hell - I'm going to do it anyway'.

Here's another example: You are on a diet and know that you shouldn't eat sweet and fattening foods. One day, you're with your friends and they're all sharing a large delicious-looking cake. You think 'Should I? Shouldn't I?'  Then you say 'Oh, what the hell!' and you take an enormous slice of cake.

Answer

I think it's similar as" hell yes !" , or" sure as hell"  , they are both slangs   used to confirm something , meaning  : very sure