To my understanding, it works like this:
your client talks to the torrent tracker, then it sends you the data about seeders and leechers. Then your client tries to connect to them, but if neither you nor the other peer have port forwarding, you cannot connect to each other.
This is not a problem for popular torrents with lots of peers, but when there are not so many it can be a problem because the other peers might as well not have port forwarding, so peers cannot connect to each other and the torrent will eventually die.
That's why it is recommended to use a VPN with port forwarding. When not using a VPN, if your router supports uPnP you are already port forwarded (with the default settings in qbittorrent).
To my understanding, it works like this: your client talks to the torrent tracker, then it sends you the data about seeders and leechers. Then your client tries to connect to them, but if neither you nor the other peer have port forwarding, you cannot connect to each other. This is not a problem for popular torrents with lots of peers, but when there are not so many it can be a problem because the other peers might as well not have port forwarding, so peers cannot connect to each other and the torrent will eventually die.
That's why it is recommended to use a VPN with port forwarding. When not using a VPN, if your router supports uPnP you are already port forwarded (with the default settings in qbittorrent).