If the application you are developing needs to connect to a local server or service, such as a REST service, you may need your virtual device to access your local host.
To do this, you can use the IP address 10.0.3.2 from the virtual device to reach your service or server.
IP 10.0.3.2 is a VirtualBox alias to your host loopback interface (i.e., 127.0.0.1 on your host machine).
For example, if you run a service on your host machine which listens to localhost:1234, it can be accessed from the virtual device using the IP 10.0.3.2:1234.
Please note, however, that this can only work if Genymotion Desktop and your server/service run on the same host machine.