The simplified explanation would be creating a space for your opponent to fill and greeting him with a punch (or combination).
There's a guy on youtube LeeWylie1 who analyses fighters (i've seen video's on GGG, Ward, Loma and Duran so far), he pretty much explains what makes/made those guys so effective. Maybe give it a try. I found it pretty informative.
Floyd vs. Hatton Floyd lets Hatton walk him into a corner and as soon as Hatton over commits Floyd hits him with a textbook check hook that drops him. This content is protected Tim Witherspoon vs. Anders Eklund Witherspoon uses his jab and steps in with each jab to close the distance on the big man and uses the last jab to turn Eklund in the direction of his overhand right for the knockout. Eklund never knew what hit him. This content is protected
Maidana kept throwing jabs to Broner's body so Broner would get used to it, then threw a jab, but turned it over into a left hook. Watch the Hopkins-Trinidad fight. Hopkins set a lot of traps there.
Maidana in particular does this a lot. He will throw a weird looping hook to your head but it will start off like he's throwing it to your body. Tricks you into defending the wrong spot. It looks very crude, but it's a good trick that he has. I miss Maidana.
Setting a trap is trying to create a situation where a boxer uses his advantages by manipulating the other boxer. So for instance trying to bait an opponent to rush in - or throw in a left hand - so when you know the fighter will do that, he'll be open to a right hand. It's one thing writing about it but to DO it and be able to technically pull it off is unbelievable. Pretty bad explanation but hey-ho! it could get way more complex than that.
If you watch Marquez' fourth fight against pacman, you could see he was setting him up all night for right hand counters, especially the overhand right. The few times he connected with the counter right resulted in pacman getting dropped and subsequently knocked out. he set them up by letting Pacman initiate the action and was timing his jabs with overhand counters. Marquez is obviously one of the great counterpunchers of the era and he's been known to set up traps on guys to get his counterpunches off. Mayweather was setting traps around the turnbuckles in his fight with Hatton. You could see from very early on (I think round 2 or 3) that when he was moving backward while Hatton was pushing closer to him, he was pivoting away and attempting to nail him with that left hook off of his backfoot while also using Hatton's momentum against him. He actually hit him a few times with it before the KO in earlier rounds, but they were grazing shots that didn't land flush.
My fav trap - slipping the jab, going downstairs an jabbng to the body. Il be obviously slipping to my right to do this so ill do it once or twice make sure to even hint right slipping reactions to their feints a bit. Then as they go to do it a few more times i slip to the left with a left uppercut lifting there head up for the right hand over the top.
Watch Sweet Pea Whitaker or Buddy McGirt to get a good clinic on trap fighting. Both were masters at it.