To equivocate is to deliberately mislead by, for example, using ambiguous language to conceal the truth. In act 2, scene 3, the porter says that drink is an equivocator because "it provokes the desire, but it takes away the performance." In other words, when one has had a bit too much to drink, the alcohol can increase one's sexual desire but at the same time decrease one's ability to perform sexually. This is of course because too much alcohol inhibits one's physical coordination. Alcohol, therefore, misleads the mind into thinking that the body is capable of having sex.
This crude example of equivocation broadly reflects the equivocation that Macbeth is guilty of throughout the play. For example, Macbeth tries to mislead everyone by trying to convince them that the king's sons murdered their father when really it was him. He also misleads Banquo by pretending to be friendly with him while at the same time plotting to have him and his son, Fleance, murdered. As Macbeth becomes more and more desperate to hold on to the throne that he took at so great a cost, he also becomes more and more of a shameless equivocator. The more drunk he gets on his own power, the more he needs to equivocate to keep hold of it.