It actually means that the Minimum Deployment Target of the included library was is to 8.3 and linking it with lower Minimum Deployment Target produces this warning.
Library does not officially support targets lower than 8.3 in this case. While linking this library to target with ower Minimum Deployment Target will still work, it might produce crashes at runtime, if any 8.3 only code is executed on a device that runs older system than 8.3.
There is a reason why developers set 8.3 as the minimum target and this warning should NOT be ignored. It might be harmless in this case, but it is not necessarily harmless in other cases!
To fix these warnings, either use an older version of the library or set your Minimum deployment target to 8.3.