I'm using airodump-ng to test my network's integrity and when I put Airodump-ng into monitoring mode, I see some networks with no ESSID and the PWR is -1. What are these networks? Are these hidden networks?
These are hidden stations. The reason there are some information missing is that these accesspoints don't send beacon packets. Beacon packets are packets that contain information about the network that is broadcasting them. This is how operation systems and applications get to know wich networks are available.
These networks bssids are shown anyway because there is traffic on that network and this traffic can't be hidden. Of cause the content of the packets might be encrypted but the protocol data isn't.
The power (-1) and quality can't be obtained because this process relies on receiving beacon packets. The beacon packets are sent in a given amount the software, airodump-ng in this case, receives these packets and rates the power and quality of the signal by how many beacons where received in if they where received correctly or corrupted.