Malcolm is the heir-apparent to the Scottish throne after the death of Duncan. He is here speaking with his brother Donalbain immediately after the discovery of the murder, which has been blamed on the king's servants -- conveniently no longer alive. Malcolm, who is quick-witted, has realized that he and Donalbain are going to be framed for the crime so that Macbeth can displace him and take the Scottish throne.
This murderous shaft that's shot
Hath not yet lighted, and our safest way
Is to avoid the aim.
Malcolm here employs a metaphor that compares Macbeth's presumed plot with an arrow that has been shot towards him and his brother. Malcolm is saying that Macbeth is sure to try to destroy them, but that Macbeth's plan has not yet been fully implemented. Instead of resisting the plan openly, Malcolm suggests, the best course of action for them is to flee, as if they were dodging an arrow that had been shot at them. Once they have dodged this "arrow," they can then decide what to do next.
Therefore to horse;
And let us not be dainty of leave-taking,
But shift away.
Malcolm here proposes they ride away as quickly as possible, without bothering to try to explain themselves or say good-bye to anyone. He obviously believes they are in such great danger that they must run for their lives.
There's warrant in that theft
Which steals itself when there's no mercy left.
Malcolm ends by warning that they can expect no mercy from Macbeth, and so they are justified in "stealing away" from the castle to preserve their own lives. Here, he uses a pun on "steal," which can mean either to take without justification, or to sneak away. In a sense, the two brothers are going to have to steal "themselves"; that is, run away without immediate justification to others, so that they can live to fight another day.
Thus, a free translation into modern English would go more or less as follows: "Macbeth is plotting to kill us, but we still have a bit of time. Let's leave before he gets us, and sneak out of here as quickly as possible, without telling anyone. There's nothing wrong with us running away from a merciless enemy who means to see us dead."