In Macbeth Act 3 scene 2, Macbeth finally accepts that "blood will have blood? what does it mean and how is it relevant to today's world?
Since the quote you are using is from Act 3, scene 4 (not 2), I will refer to that scene in my answer.
The complete quote is:
"blood will have blood / Stones have been known to move, and trees to speak / Augures, and understood relations, have / By magot-pies, and choughs, and rooks, brought forth / The secret'st man of blood".
Macbeth is the "secret'st man of blood", that is, he is a murderer whose crimes have not yet been discovered. However, he is aware that he will not get away with it. By saying that "blood will have blood", he means that the blood of a murder victim will seek out the blood of their murderer, and thus a murder will always be brought to light (see Shakespeare navigator on blood imagery).
Macbeth has murdered Duncan and Banquo. The moving stones can be interpreted as
a) the stones that covered the bodies, so moving stones will bring about the discovery of their bodies, which is the first step to discovering the identity of the murderer (see Arden Edition, p. 96)
b) or the "stones of judgment" by which the Druids tested the guilt or innocence of a suspect (Paton 1869).
Furthermore, "blood will have blood" means that Macbeth will not be able to shed an innocent's blood and get away with his crime unharmed - his life will be demanded from him. The whole play is dominated by bloodshed, and Macbeth will be decapitated by the end of the play. It is a barbaric world in which sheer force and violence rule, in which one bloody crime is avenged by another.
The relevance of this quote for today's society could therefore be seen in the fact that violence will always breed more violence. As the Irish folk singer Tommy Sands famously phrased it in his song "There were roses": "An Eye for an Eye / Another eye for another eye till everyone is blind." (see link to his song below)
Those who break the rules must be punished, yet the question arises whether to re-establish the law by doing more violence. In today's world, if Macbeth's quote is to be taken literally, war cannot be but the last resort when all diplomacy fails. The vicious circle of violence can only be broken if blood will not automatically want more blood.