The stock answer is that it is about suicide. The problem is that it doesn't make any sense as being Hamlet contemplating doing away with himself. The speech is not phrased as being about Hamlet personally--the words "I", "me" and "my" do not appear in it. Instead it is "we", "us" and "our". Hamlet has already dismissed the idea of suicide for himself in Act 1, and he has just finished deciding on vigorous action just before giving this speech. He is not in a suicidal mood.
It seems to be more of a specualtion about why people are "patient Grissels" instead of trying to do something about the miseries of their life, even by committing suicide ("their own quietus make with a bare bodkin"). He comes to the conclusion that the fear of death makes cowards of people generally, making them suffer rather than do something that would change their condition.
The setting for Act 5 Scene 1 of Hamlet is a graveyard. This sorts with the theme of death which has been flowing through the play.
The Ghost, in Hamlet, Act 1 Scene 5
Hamlet. See http://www.opensourceshakespeare.org/views/plays/play_view.php?WorkID=hamlet&Act=1&Scene=1&Scope=scene
Hamlet, Act 3 Scene 1
Perhaps the line you are thinking of is Hamlet's line in Act III Scene 1: "We are arrant knaves all; believe none of us."
The setting for Act 5 Scene 1 of Hamlet is a graveyard. This sorts with the theme of death which has been flowing through the play.
Hamlet, Act 3, Scene 1.
In Act 1 Scene 4
Hamlet - questioning the meaning of life
The Ghost, in Hamlet, Act 1 Scene 5
Horatio plans to go meet Hamlet at Elsinore, which he does in Act 1 Scene 2. Horatio plans to meet Hamlet on the battlements, which he does in Act I Scene 4. Horatio plans to meet Hamlet at the play, and does in Act 3 Scene 2. Horatio has no plans to meet Hamlet in England.
Hamlet. See http://www.opensourceshakespeare.org/views/plays/play_view.php?WorkID=hamlet&Act=1&Scene=1&Scope=scene
act 3 scene 1 lines 147-148 :)
Horatio, in Act 1 scene 1. Hamlet doesn't see it until scene 4. It has to be this way because in scene 2 Horatio tells Hamlet that he has seen the Ghost.
In the exposition of Act 1 Scene 5 in Hamlet, two characters speak: the ghost of King Hamlet and Prince Hamlet. The ghost reveals the circumstances of his death to Hamlet, setting the tone for the rest of the play.
These are the first six words of a speech Hamlet makes in Act 3 Scene 1 of the play.
Hamlet Act 3 Scene 1