You've already been told to read the FAQ but here, I'll explain it to you since reading webpages is apparently a bit complicated for your mind.
You can achieve two sorts of connections to peers when you torrent something: incoming, and outgoing. An incoming connection means when the connection was initiated by the other user - this means that your network has to be properly configured to allow this unsolicited request to reach your machine, and thus your torrent client, for your computer to respond and make the connection. And outgoing connection is when your computer attempts to make the connection with one of the peers it has been told about from the tracker etc. by sending them a connection request.
Now, if your router or firewall is configured so that incoming requests cannot reach your computer, the only sort of connection you can make is an outgoing connection - and you won't be able to make outgoing connections with anyone else out there who also has an improperly configured network since they won't be able to receive your request. There are so many people with improperly configured networks out there that being unable to connect to them effectively isolates you from a large proportion of the people who are on the torrent, meaning you get slower speeds overall. If you can receive incoming connections, you can theoretically connect to every peer or seed on a torrent, making your potential speeds much higher.