Understanding the rules of Knock-out Whist
Q: Trump suit here means that even if the category of the card is lowest, it will be considered higher than ACE too?

A. Yes. By category, you mean Rank.  Any card in the Trump suit is stronger than any non-trump suit card.

Q: What is the "first trick"? What is the meaning of "card may be led"?

A. in a trick each player play 1 card.  The stronger of the 4 cards wins the trick.  Leading the trick is the first card in the trick

Q: What is the meaning of "discarding or trumping"?  

A. Players must follow the leading suit, however when they don't have that suit in their hand they can either play another non-trump suit and lose that trick for sure (discarding) or play a trump card (trumping)

Q: "unless a trump is played" - What is the trump in this case?


  one card is turned up to indicate that its suit is the trump suit for
  the first round


Q: What is a hand?
A. the cards a player is holding

Q: What is the meaning of "player who took no tricks"?
A. player that didn't win any trick when the round is over (all cards in hands where played).

Those are general Trick-Taking-Game questions.  I would start by reading the rules of simpler trick-taking card games like Whist and Spades, see https://www.pagat.com/whist/whist.html