NOTE: from firmware 2.2.0, OpenSprinkler 3.x will support remote access via OpenThings Cloud (OTC) connection, thus it doesn't need port forwarding anymore. Firmware 2.2.0 has been officially released.
If you are using OpenSprinkler app version 1.3.1 or newer you might notice a notification saying remote access is not enabled. This means the OpenSprinkler cannot be accessed from outside your home such as work, hotel, etc. In order to allow this access, you must configure your router to allow a port to be routed to the OpenSprinkler and grant it access to communicate.
Port forwarding is simply telling the router that anything coming on port 80 (default port) to be routed to the OpenSprinkler IP (eg. 192.168.1.22). So in order to port forward you will need to know this IP address which can be found on the LCD by clicking B1. Also, the application is capable of scanning the local network and identifying devices which also shows you the IP.
The OpenSprinkler IP is assigned automatically by the router and can sometimes change which will prevent the port forwarding from working. In order to prevent this, you may setup a static DHCP on the router which assigns the same IP each time to the OpenSprinkler. In order to do this the router needs to be told the device MAC address which can be found on the LCD by clicking B2.
For details on your specific router please consult the router manual and look for "static dhcp" and "port forwarding". The above video demonstrates the most common brands which covers a large majority of use cases.