In canon (not 100% sure how much got kept into MfD, though I have the impression it's mostly the same) it was a seal invented by the 4th Hokage (the one who died sealing the 9-tails) which allowed him to instantaneously teleport to any copy of it in existence, and then he did things like give a knife with one on it to each ninja in an army and teleport-spam to take out whole battalions by himself. Nobody since has managed to figure out how it works. It was enough to get him a flee-on-sight order from basically everyone, because with the all the flickering about he was functionally untouchable and could just pop around until he got into your blind spot, then kill you.
What he said. It's a legendary seal, and if Jiraya hasn't managed to figure it out yet (despite being the guy who taught the 4th and also very likely knowing more about the seal than anyone else alive) I doubt we have a chance.
Edit: I have been reminded that I should be fully pedantic, and say that the 2nd Hokage invented the base seal, but the 4th was the one who adapted and weaponized it into the modern, horrifying form.