From Hamlet's soliloquy "To be or not to be"

To be -> to live -> it means to suffer the injustices of life, such as the "heart-ache" and "the thousand natural shocks that flesh is heir to". Hamlet makes a list of all the troubles one has to face as he decides to keep on living:
• Whips and scorns of time
• Oppressors' wrongs
• Proud man contumely
• Pains of despised love
• Scorn of time
• Delay in applying the laws
• Insolence of powerful people towards subjects
• Disdain of unworthy people towards the merits of patient people

A man may commit suicide to escape these troubles, but he has to face the fear of something after death (not to be).

Not to be -> to commit suicide -> it means to oppose the troubles of life, ending with them. Death is initially compared to a sleep, a consumption which however comes pleasant. -> In a sleep there may be unexpected dreams or nightmares, and in the "sleep of death" there may be something we do not know anything of. -> Men prefer to stand the difficulties of life, since pains after death ("the undiscovered country, from whose bourn no traveler returns") may be worse -> every great action vanishes or becomes weaker ("the native hue of resolution is sicklied o'er") in front of death