Explain with reference to context the following lines from Hamlet.'Tis now the very witching time of night, When churchyards yawn and hell itself...

In Shakespeare's Hamlet, the lines you quote are from Act 3.2.356-360. 

Before Hamlet speaks these lines, he gets the proof he's been waiting for that the Ghost is telling him the truth and that Claudius is definitely guilty of assassinating King Hamlet. 

In these lines Hamlet emphatically claims that he is now ready for action.  He's ready to get revenge and kill Claudius.

Specifically, Hamlet says that it is the witching hour, the dead of night, the time of night when graves and graveyards open and hell breathes stench upon the world.  Now could he drink hot blood and do things (like kill Claudius) that the day would be afraid to look at. 

Of course, Hamlet doesn't immediately act and get his revenge.  He has an opportunity when Claudius is alone and praying, but Hamlet does not want to send Claudius to heaven by killing him just after he's confessed his sins.  By failing at this point, Hamlet brings about the catastrophe at the conclusion of the play, resulting in many more deaths than necessary.