[Footnote II.3: Ecstacy of love;] i.e., madness of love. In this sense the word is now obsolete.
[Footnote II.4:
This must be known; which being kept close, might move
More grief to hide than hate to utter love.
i.e., this must be made known to the king, for (being kept secret) the hiding Hamlet's love might occasion more mischief to us from him and the queen, than the uttering or revealing of it will occasion hate and resentment from Hamlet.
It was the custom of Shakespeare's age, to conclude acts and scenes with a couplet, a custom which was continued for nearly a century afterwards.