In the Name of God, the Compassionate, the Merciful.
Hi!
Conversation
A: Were you able to attend Friday night’s basketball game?
B: I was unable to make it.
A: You should have been there. It was intense.
B: Is that right. Who ended up winning?
A: Our team was victorious.
B: I wish I was free that night. I’m kind of mad that I didn’t go.
A: It was a great game.
What does “I’m kind of mad” mean?
Thank you
This makes more sense if they said:
I’m kind of mad (or angry) that I couldn’t go.
They had another obligation at the time of the game. That is what they are angry about.
Didn’t go
vs
Couldn’t go
Didn’t go in this context implies that it was their choice to not go.
Couldn’t go implies that there is some other reason. They did not have a choice.
Kind of means a little bit mad or somewhat mad.
I’m kind of hungry.
I’m kind of tired.
Thank you so much, NearlyNapping 
Very nice.
Is ‘kind of’ a filler like ‘you know’, ‘I mean’ etc.?
As NearlyNapping says, ‘kind of’ is a modifier like ‘a little bit’ or ‘somewhat’, so it’s not a filler:
Thank you, Torsten.
A fruit is a kind of food.
( Or type / variety / sort of food )
These words can also be used as degree or relative modifiers, as well as distinctly different types of something.
Somewhat mad - (Kind of mad)
Mad
Very mad
A little cold - (Kind of cold)
Cold
Very cold
‘Kind of’ usually means a little, or somewhat.
An exception is:
I’m kind of busy.
This probably means very busy. This takes on a meaning of not wanting to be interrupted, not having the time for something, being distracted, etc.
A: Can you help with my homework?
B: I’m kinda busy right now. I can help you this evening if that’s OK.
A: Did you hear what I said?
B) No. Sorry, I was kind of busy. (distracted)
As is often the case, the words ‘of’ and ‘to’ are often contracted.
Kind of → Kinda
Sort of → Sorta
Going to → Gonna
Want to → Wanna
Those are all in the dictionary as informal contractions.
Very nice.