To understand the quotation, it must be considered in context. Hamlet is sickened by his mother's marriage to Claudius, viewing their relationship as incestuous. His rage has grown, and he plans to confront her about her betrayal of her love for Old Hamlet and her sinful behavior with Claudius. Before entering her chamber, Hamlet anticipates what will occur--and must not occur:
O heart, lose not thy nature; let not ever
The soul of Nero enter this firm bosom.
Let me be cruel, not unnatural;
I will speak daggers to her, but use none.
My tongue and soul in this will be hypocrites.
Hamlet intends to speak with cruelty to his mother, but he loves her. He must not lose control and harm her. To do so would violate the natural love between them. He will "speak daggers," but he will use no weapon against her. His words and actions (his "tongue and soul") will be hypocritical, therefore, since they will not reflect his deep love for his mother; he will not mean everything he is about to say.