The quote represents King Duncan's words when the king realizes that the Thane of Cawdor has worked in collusion with the Norwegians in order to help them defeat Duncan. The king strips the thane of his position at once, and, as a result, Macbeth is promoted for his valiance and loyalty to the king:
What he hath lost noble Macbeth hath won.
The quote demonstrates that the king has a very high opinion of Macbeth and values qualities such as valor, loyalty, and true friendship. He wants to honor those who exhibit such qualities, and we see him placing his hopes into Macbeth, believing Macbeth is one of his biggest supporters.
The tragic tone starts to permeate the play once we realize that Macbeth is ready to abuse the trust which king Duncan has put in him for the sake of fulling his own evil ambitions. Although he has every reason to protect the king, Macbeth dares to create disorder by plotting to kill him so that he can become the king himself. He murders Duncan, but that very action will ultimately result in Macbeth's own downfall.