Let us remember what has happened in the play up until this stage. Romeo and Juliet have married in secret, then Tybalt was killed by Romeo, resulting in Romeo's banishment. Juliet's parents have insisted that Juliet marries Paris the next day, so Juliet went to Friar Lawrence, threatening to kill himself. Friar Lawrence manages to dissuade her from this self-destructive action by coming up with a plan. He gives her a potion that will give her the appearance of having died whilst in reality only reducing her to a death-like state for a given period of time. She will be taken to the Capulet family vault, and Friar Lawrence will tell Romeo so that he can come back and wake her from her sleep.
In this scene, therefore, Juliet has a lot on her mind. She is due to be married the next day to Paris, and has to take this potion that she hopes will save her from this fate. However, she is left with great fear about what should happen if the potion doesn't work, or if it actually kills her. These issues are presented in the soliloquy that Juliet gives in this scene.
Juliet is being forced to marry Paris by her father. She hopes the potion the friar gave with work and the wedding won't happen.