"Et tu Brute" is one of the most famous and requoted lines from Shakespeare. It typically is translated to mean, "You too Brutus?" or "And You, Brutus?"
Caeser says this as he recognizes his close friend, Brutus, taking part in Caeser's assassination. It is Caeser calling out his close friend, who has turned on his emperor friend in search of power for himself. This ambition and greed overcomes their once close friendship. Once Caeser sees that even his good friend Brutus is taking part, he gives up and dies.
Another idea is that instead of Caeser being saddened and shocked by his friend Brutus' particpation, he is actually damning his friend by saying, "Your turn will come" or "Damn you to hell too." This is fitting, because like many in power, Brutus also eventually dies in a similarly violent manner.