If you don't know how to do port-forwarding, go to Port Forward, search your router, and follow the instructions. If you are setting up PocketMine-MP on a home network, you have to port forward the PocketMine-MP UDP (and TCP if using RCON) port on your router.
You can check all the settings and their values on the server.properties page on the Wiki. In the folder where PocketMine-MP has been installed, open the server.properties file with a normal text editor. To set up a server, you need a supported device.
