What was Big Willy Shakes going for?
Well, in the literal sense, the porter is asking who is at the door. But we're guessing you knew that already. It's Macduff and Lennox, who have come to fetch the king. (He's already dead so they'll go home empty-handed.)
But in the figurative sense, there is something much darker happening here. The Porter says maybe there's a Catholic "equivocator" at the door "who committed treason enough for God's sake" (2.3.1).
On the one hand, an "equivocator" is a person who speaks ambiguously or doesn't tell the whole truth, which shows up over and over in Macbeth. Macbeth frequently bends the truth as he deliberates about whether or not it's okay to murder the king; he equivocates when he justifies (to his henchmen) that murdering Banquo is acceptable; and even Banquo has some ambiguous thoughts about the prophecy that he'll father kings.
On the other hand, the word "equivocator" is most likely an allusion to and old treatise written by the Jesuit Henry Garnet, who encouraged Catholics to speak ambiguously or, "equivocate" when they were being questioned by Protestant inquisitors (so they wouldn't be persecuted for their religious beliefs).
What does that have to do with anything? Henry Garnet was tried and executed for his role in the Gunpowder plot of 1605, when a group of Catholics planned to blow up the King and Parliament (they stored kegs of gunpowder in a nearby building). The plot failed, but it was a majorly upsetting experience for everyone involved.
It's likely that a lot of Shakespeare's audience members would have associated the scene right before this (where Macbeth returns from murdering the king) with this terrorist plot. And if they didn't before, the Porter certainly helps them make this connection by calling an "equivocator" to mind.
Ooh. We just got chills.