It is always important when you are trying to work out the meaning of quotations to read them in the context of the entire work of literature. Often this will help reveal the meaning of the quotation. This quote, uttered by Macbeth, is said in Act III scene 4 at the end of the famous banquet scene, after Macbeth has been greatly disturbed by the sight of the ghost of Banquo, whom he has just had killed. The quote you have identified reveals Macbeth's own perceptions of where he is morally. Having committed so many nefarious acts, such as the murder of Duncan and now the murder of Banquo, he considers himself to be beyond the pale of redemption. He is so deep in the blood of the innocents that he has killed, that even if he were to not "wade" any further, it would be too much work to go back and right the wrongs he has committed. Therefore, he might as well continue on his path to damnation, adding evil to evil. This quote is therefore very important when we think of Macbeth's state as a character. It represents his own abandonment to evil and to the damnation he will receive.
Macbeth has stepped into the pool of blood so far in, that it wouldn't matter whether he stepped forth or back, it would be the equal dist. both.... Therefore, he is to remain where he is....