These words of Francisco come at th every beginning of the play, when Barnardo arrives to relieve him from his position of being on watch in Elsinore castle. When Barnardo tells Francisco to get to bed as he has arrived to take his place, he responds saying the following words:
For this relief much thanks. 'Tis bitter cold,
And I am sick at heart.
Even though Francisco leaves the stage soon after uttering these words, and never is seen again in the play, these words play an important part in raising the tension and also foreshadowing what is to occur later on in this scene. The fact that Francisco, clearly a seasoned soldier, is "sick at heart," causes the audience to question what is going on and why a soldier should be so disturbed by being on watch, which would have been a normal duty. This leads the audience to suspect that something else is going on, some supernatural event perhaps, that is impacting Francisco so deeply. This of course foreshadows the appearance of the Ghost.