"You should know, my noble son, the real snake that stung your father is now wearing his crown"
"I knew it! My uncle?
"Lord Hamlet-"
"Heaven secure him!"
"So be it.
In this scene of the play, the ghost tells Hamlet that he did not die on his own, someone had killed him and he tells Hamlet to get revenge for his murder.
"What is it , sir? Of course we will"
" Never make known what you seen tonight"
The ghost tells Hamlet that the person who killed him, is the new king (claudius) of the country
" You swear never to mention what you've seen . Swear by my sword"
" We won't sir"
After the ghost left to his underworld again, Horatio and Marcellus showed up and they see Hamlet, standing there alone. They are worry about him and asked him if he is okay!
" (beneath) Swear."
Hamlet tells Horatio and Marcellus that they can't tell anybody about what they saw and heard, while the ghost was there.
Horatio and Marcellus swear that they will not tell anyone about the night event of the ghost.
The ghost of King Hamlet is watching them from under the stage and he tries to "make" them swear that they will not say nothing.