what is meant by the seal of the confesional?

Answer 1

It means the priests are bound by an oath, a seal, of silence. They can never reveal what is told to them in confession, not even to a judge in a courtroom or law enforcement office. If they break this seal, they could be excommunicated from the Church and stripped of their priestly faculties. As you can see, this is a very big deal.

Answer 2

It means that what is said there stays there. Kinda like Vegas.

Here is a link if you want a more indepth answer:
http://www.newadvent.org/cathen/13649b.htm