They are indicators and they change depending on the connection status:
The green line indicates that the device is connected to your mobile. The grey line with a rotating circle indicates that your device is close to you and the mobile tries to connect to it. The orange lines indicate the devices set as Not-to-Forget (N-t-F). Initially these devices are not connected. To force the connection to a N-t-F device, go to the list where your devices are displayed and select the device. After that you can press the "Call" button and the device will start ringing. The grey line indicates that the device is out of range. You will be able to check the exact place and time it was inside range for the last time. The grey line with a sad emoji face indicates that the device is out of range and you have reported it as lost. The grey line with a smile emoji face shows that your lost device has been found by the SpotyPal Community. You can now check your notifications to check the place it was found.