"To be, or not to be" is arguably William Shakespeare's most famous line. Found in Hamlet (3.1.56), Hamlet is considering life. He finds that life may not be worth living if it continues to be so challenging. Hamlet is questioning whether he should move forward with his plan to murder Claudius (the one responsible for his father's death) or end his own life.
"To be" refers to the verb "being" in regards to existence. Hamlet is essentially questioning if he should continue his own insufferable existence or end his pain. Unfortunately, Hamlet also questions what the afterlife holds. His fears of the unknown force him to consider which is the "lesser of two evils" (when given two bad choices, one tends to be "less bad" than the other), ending the life of Claudius or his own.